I can't use optifine or forge.

When I use Optifine or Forge for minecraft, it DOESN'T WORK! After I press play, and after it has prepared, the  loading screen is supposed to pop up. But... when I try, nothing happens! Well, minecraft thinks something happens, because, when I press play again it says that there is already a game running. 

Please, fix this!

 

Edit:

I have read what you said. You wrote that I had to open this file in my minecraft folder.. Well, I don't have that file! I've searched through all my folders, but I can't find it.. Do you think you can explain in detail where to find it?

 

I also have another problem which only happens when I don't use forge/optifine and that is that:

After I press play, that red bar and the text mojang pops up, right? Well, when that bar has loaded to about 90% it just stops. And a few seconds later, the program says ''not responding'' could you please fix this too?

(The weird thing is that it only happens 9/10 out of the time).