"I downloaded the roster file offline" - this might be your problem... what exactly is that file?
Two things.... if the roster file is an official DLC update for the game, that content goes in the HDD1:\Content\00000000 folder.... the DLC file itself should be within a folder with numbers and letters (e.g. 5454D811) and then a 002 directory within that... so if thats the case, thats part 1
Secondly, alot of games have TU's (title updates) which are required to recognize new DLC updates. Sometimes these are included in the DLC, other times you need to search for them... just google xbox 360 TU database or something like that... look for the latest one for your game (you dont need them all, just the latest one). Put that file (starts with TU and then a bunch of letters/numbers) in the HDD1:\Cache folder....
Restart your xbox and ideally your updated roster will be there. Worst case if you love the game go buy it along with a retail 360
