Trying to find out if I need to have the microwave vented to the outside

Yes, an adapter/kit should have come with the unit and then you would have to buy the piping etc.If you do not have the adapter/kit get it from the manufacturer or unit seller.

I usually choose an outside wall if possible to vent outside. You can buy a rear vent that is superior to a dryer vent. When this isn't possible you need a rectangular duct going up through your cabinet and up through your ceiling. hopefully you have an attic or you have to switch to a 5 inch tube going sideways (above your cabinets (if there is space) to the outside wall. If it goes into an attic you have to cut through the shingles to a vent made for bathroom and or hood vents. The motor and blower assembly inside your microwave is designed to turn to be front facing, rear facing or vertical facing. Just unscrew or break out the side that you need to vent through.

If there is a vent outside it will be behind micro,you have to take it down to check that 1 st. If there is the motor must be removed and reinstalled to redirect air flow to outside.

When you say "outside vent" do you mean horizontally through an outside wall? If that is what you are trying to do having the 2x4 in the way does not make much of a deterrent. But if it makes you feel better, moving over a bit and not have the 2x4 in the way, that certainly would be better.

Interesting post, because I have the same problem! I have spent the better part of today researching and taking our microwave apart (partially) to inspect the top portion re: outside vent. Everytime the microwave is on, the "active vent door" (manufacturer's term, not mine!) opens. Apparently this is normal. But it also opens when you turn on the vent fan, when cooking on the stovetop. I don't like this feature, because I think it 'overrides' the outside venting system. I double checked our outside vent to ensure that when this oven was installed, the person did in fact allow the flap to be open when the fan is on (he did). So I think what 'our' problem is, is that the microwave active vent sort of interferes with the outside vent, thus more of the cooking smell being blown into the house instead of the outdoors. I was thinking of simply disconnecting the gizmo that make the active vent door open each time you use the oven and/or vent fan option however I don't think that's a good idea as apparently the main purpose of the active vent door is to keep the electronics of the microwave oven from overheating. The more I read about this version of microwave, the less impressed I am with it. Lots of electronic issues with it. I am not fussy either with having the microwave over the cooktop. Sorry I don't have a solution for your problem but I do feel your pain!!!

Yes, this microwave will vent either out through the top or the back. A damper will be needed if it was not saved when the unit was installed. A list of other items are listed in the installation instructions at this link To GE.
http://products.geappliances.com/MarketingObjectRetrieval/Dispatcher?RequestType=PDF&Name=49-40409-1.PDF

I would say that you do not have to vent it outside. One of the most common complaints I get is that the microwave is not sucking up a lot of air. I have called Whirlpool and they have said that most of these newer microwaves don't really have a lot of air movement in them and they have smaller blower motors. Some of this was due to the fact that people didn't like the noise of a big fan that was necessary fo move air. I have noticed this on my own also. When boiling sweetcorn this summer, it would barely **** up the steam whether it was recirc or to the outside, I usually test them by putting a cleenex under the vent abd see if it holds it there. Bottom line is they really don't move a lot of air and it's not worth the cost and time of venting it out. Hope that helps

Your vent flap could be taped or it could be faulty installation not allowing the vent to open. If you take the front vent cover off, you should be able to reach in above the fan unit and gently push the vent flap up, it should move freely with no binding. If it doesn't it is taped or blocked. If it is taped, gently putting upward pressure on the flap might release the tape. If that doesn't work the vent installation is blocking the flap. You might have to remove the unit from the wall (two people to do this) or disassemble the vent duct above the microwave and rework the interface for clearance.

hope this helps,

You will need the take the microwave down and take the shell off. Then take the vent fan out and turn it around the opposite direction and it will blow the other way.

Yes this can be done.  The unit will need to be taken down and the vent fan will need to be reversed.