Do you know which version you downloaded? Check the PS Store description, and check the BLUS code on your case. Whichever DLC matches your code is the correct one.
If you did download the wrong one, try contacting Sony, I think they have a new department for Network/Store stuff now. Just patiently and politely explained what happened, and they should fix it for you. I think.
If that's not the problem, try going into safe mode and do a "file system restore." this will clean up your HDD and get rid of any corrupted data, keeping everything else. After this, try re-downloading & installing the DLC.
I'll be back with a link to explain getting into safe mode.
Edit: Here it is. Make sure you "restore file system" and nothing more. There's an option or two on the safe mode menu you really don't wanna press.
http://community.us.playstation.com/thread/627262?start=0&tstart=0
Hope something helps.