How do I get into the filter to check backwash valve?

FILTER is the valve position when the system is in the normal filtering mode.
When you backwash the filter you first need to turn off the pump, place in BACKWASH position, and run the motor for 45 seconds (or until the backwash water is clear). Turn off pump and run in the FILTER position for 15 seconds. Repeat twice more.
The RINSE position is to clear any dirt in initial start-up cleaning, plus resetting filter bed after backwashing.
The WASTE position is for vacuuming directly to waste, lowering pool level, or draining the pool.
The CLOSED position is for shutting off all flow through the valve.
The RECIRCULATE position allows water to flow from the pump through the valve and directly to the pool.

You will probably only to use the FILTER, BACKWASH and maybe the RINSE valve position. The RINSE position is not as important as indicated by the fact that many backwash valves don't have a RINSE position.

I hope this helps.

check that the valve is fully closed
replace that valve

After your pool filters on the "backwash" setting, you should move it to the "rinse" setting. With the pump off, set valve to rinse. Start pump and operate for about 1/2 to 1 minute. This ensures that all dirty water from backwashing is rinsed out of the filter to waste, preventing possible return to the pool. Stop pump, set valve to "filter", and start pump for normal filtering. If it still won't start, check for restrictions in the intake and discharge lines.

You posted this question in Alarm category. but sounds like either your filter laterals, or the backwash valve are bad. I'd need to know what type of filter, and backwash valve you have. is the backwash valve a multiport, or push/pull slide valve, etc... If it's a push/pull valve then the o-rings, or pucks can be bad, or the piston can be broken. if it's a multiport valve, then the spider gasket may need to be replaced. if it's a top mount multiport valve, then it can be broken inside the filter.

backwash the filter from main drain until sight glass is clear-this will reduce filter pressure. After backwashing return gate valve to filter position, then filter from main drain only. check water chemistry. add a clarifier to water

For instructions on backwashing and other trouble shooting, you need an Owner's Manual. You can find one online at the following link:

http://www.pentairpool.com/pdfs/FNSPlusVerticalGridDEFilterOM.pdf

The way you backwash this filter is easy once you get the hang of it!

The first thing you will do is turn off the system, change the valve position to the Backwash position. If you have a Multiport Valve, you will press down on the handle and spin it so that the knotch is pointed to BACKWASH. If you have a Slide Valve, only has two positions, up and down, you turn the handle so that the metal pin is out of the holder and move the handle in the opposite direction. You will need to turn the handle to lock the pin into position.

Once this is done, you will turn on your pump. Pentair recommends that you backwash for 1 minute, then turn off the pump.

If you have the Slide Valve, you will put it back into the FILTER position and then turn the pump back on for 1 minute. If you have the Multiport Valve, you will put it in the RINSE position and then turn the pump back on for 1 minute.

Pentair recommends that you do this cycle a total of three times, backwash/rinse, backwash/rinse, backwash/rinse. AFTER the third time, you will need to open the Air Release Valve at the top of the filter while the pump is OFF and the valve position in back on FILTER. Once this is done, turn on the pump. When you have a steady stream of water coming out of the Air Release Valve, close that up. This way you have no air in the system.

Now, you will mix your DE "Slurry" in a bucket. Depending on your model of filter, you will measure out your DE and mix with water to the consistancy of milk. **IF it is just a backwash, you will only add 80% of the original amount of DE. You only add the total 100% of DE when you completely take the filter apart and clean it out.**

You will slowly pour the slurry into the skimmer that is closest to the pump. This will allow the DE to evenly coat the grids in the filter. Wait about 5 minutes and then check the pressure gauge on the filter. Once the PSI reaches 8 to 10 PSI above this number, it is time to backwash.

Check the pump suction filter? To backwash it, is to
open the backwash tank cleanout valve, to clear the dirt out. If this does not work, check the pipelines going to the backwash tank for clogged lines.