I just finished going though this myself, seeing all of the great help online with fixes that probably do work for some was getting frustrating. But nothing was working for me.
It sounds simple, but the way to get boinc communicating with your cards is to get rid of all of your previous driver software (if possible) and manually install the newest drivers from the website.
This requires downloading the setup.run file, making that file executable (right click, go to permissions or sudo chmod -x filename.run) and then booting into recovery mode and executing the setup (go to directory and ./filename.run) have it register with dkms but do not change your default desktop (unless you want to). I was unable to remove my previous drivers or install the new ones manually, something got corrupted with the ppa’s and a fresh install was needed, which might be the case for you too if you have been trying to get it working with numerous setups.
After I installed the drivers I just installed boinc through package manager, nothing special, and it loaded and recognized the gpu’s! I was good to go!
I am currently running debian 7.8 xfce but it did also for me on a FRESH Linux Mint 17.1 install, which was what I started with and had all the problems the first time around. With the ubuntu distros like Mint it seems to have the most trouble, and usually due to having opened or messed with the proprietary hardware driver selection app that is pre-installed on the system. Since I had the driver pre-downlaoded I forced it in before first boot and it worked fine. Also just installing the main boinc program, no mods or extra downloads.