Please respond ASAP - I won't have Internet access after 40 minutes!

My model is black with a white cap.

The toilet won't stop running unless I lift the arm on the
Fluidmaster, after which it seems to be fine.

So I checked out the instructions for that online, and it said:

Problem: Debris is lodged under the valve's seal. Debris may consist
of calcium deposits, minute pieces of corroded pipe, rocks, rubber,
solder, etc. that are carried by the water stream to the seal area,
where they become trapped. Some well systems without a filter
constantly circulate debris.

Solution: Shut off the water supply and remove the valve top (Model
400A: lift arm first). Cover the opening with an inverted cup, and
turn water on and off a few times. This should flush and remove water
line debris. Turn top assembly over and clean seal surface by rinsing
under water. Replace top (400A: lift arm first) by rotating
clockwise.

(end)

Nice, except that when I tried it, I couldn't get the cap off (yes, I
twisted counterclockwise). Prying didn't do a thing either. I'm afraid
I'll break it if I put any more pressure. What am I doing wrong?

Granted, another possible solution was listed here, which I'll try in
a few hours:

http://www.fluidmaster.com/html/troubleshooting.html#q5

Problem: The water level is set too high and reaches the top of the
overflow pipe upon shut-off. This results in small amounts of water
pouring into the overflow pipe, causing the valve to turn on to refill
the tank.

Solution: Reset tank water to a lower level by using the water level
adjustment clip (see illustration) located on the link of the valve.
Squeeze the two sides of the metal clip together, then slide clip down
the rod.

Again, please help ASAP - the plumbers are inspecting the building
tomorrow and I'd rather do it myself. Thanks.

Lenona.