Tumble arm inside fell off. How do I rescrew it??

You will need to disassemble the dryer to get to the drum to reinstall the tumbler arm...
I will include a link that shows you how to hook up the belt.
I will include the information for repairing the belt, just when you get to the drum removal, that is as far as you will need to go unless you want to lube your drum rollers to stop future problems.

http://www.applianceaid.com/images/belt-config.JPG

If you hear a loud bang while your dryer is working, and you notice that the
drum has stopped turning, chances are the drivebelt has snapped.
Fortunately, replacing the belt is a fairly straightforward operation that
can be handled with a few simple tools.
First, unplug the dryer. Lift the lint-screen cover and pull out the lint screen
If your dryer has a lint screen on the inside of the door, skip this step.
Then remove the two screws that are under the cover. To unlatch the dryer top,
insert a thin-blade putty knife between the top and front panel,
about 2 1/2 in. in from one corner. Push in on the knife to release the locking
clip, lift the corner, then release the clip on the other side. Swing the top
up and lean it against the wall.
Next, move to the lower toe panel. Slip the putty knife between the lower panel
and the door panel at one side and twist the knife to release the clip
Then release the opposite clip and lift the panel up and off the hinge springs.
To support the dryer drum while the belt is being replaced, place scrap wood
under the front of the drum.
Loosen the two screws at each side of the bottom of the front panel.
Move to the top of the panel and remove the screws from the inside of the
cabinet that hold the front panel to the sides.
Slide the old belt off the drum.
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Installing the belt
Lean the front panel out slightly and allow the drum to rest on the scrap wood
support. Drop the new dryer belt between the front panel and the drum, and
loop it over the drum. Then, lift the drum slightly while pushing the front
panel back into position.
Replace the screws at the top inside of the front panel, tighten the lower panel
screws and remove the scrap wood from under the drum. Check to make sure that
the rear drum seal fits properly over the lip on the rear bulkhead.
Position the new belt where the old belt was, with the grooved side of the belt
facing down.
Now, move to the motor area and pull out the old belt. To route the new belt
around the idler and drive pulley, first pass the entire belt, doubled up,
under the idler pulley. Then, push the idler belt toward the motor, slip the
belt around the motor pulley and release the tension on the idler pulley.

Open the dryer door and rotate the drum counterclockwise a few times to make
sure the belt is tracking properly. Lower the top and press down at each
corner to engage the clips. Lift the lint-screen cover, replace the two screws
and reinstall the lint screen. Finally, secure the lower front panel and turn
on the dryer for a test.
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