restarting your computer has to restart steam.
After a fresh bootup, when you launch steam what happens?
If it opens ok but theres no "digital paintball source alpha 2" in the list.
Its probably means the installer put the files in the wrong place for you.
Are your steam games at;
" C:\Program files\Steam\Steamaps"
In this directory you should have a "sourcemods" folder. Within that is where the "dpb" folder should be to appear in steam and work.