Hopefully they didnt charge you for retropie as well as the rpi2, cause retropie is free and quite simple to setup. (Plus its unlikely it has the latest image as that was just released today.) I’m not sure what to tell you on the video issues as I’ve never had that problem, could be an issue with your hardware or if you bought the pi secondhand it could have also had problems from the previous owner. You could try installing a new image on it (retropie 3 beta 4) and see if that works. (You’ll probably want to make a backup of your SD card first just in case)
As far as mame goes it can be a bit confusing at first to get sorted out as different emulators use different romsets even if they are the same game. You’ll likely have to rebuild your romset using a program called clrmamepro so they will be compatible with your retropie’s version of mame. See this post and accompanying video:
The retropie wiki should answer a lot of your questions if you’re just getting into retropie, it may be a bit of a learning curve at first but its worth it. I try my best to keep the wiki as updated as possible so that it is useful to newcomers especially.