Borderlands 2 downloadable character problems?

The characters you downloaded were probably meant for PC, not 360. The 360 does not allow external saves like that, as saves are tied to your gamertag. Also, a flash drive will not work in a 360 until it is formatted specifically for the 360.

Unfortunately, there is no way to get a head start unless you find someone willing to boost you. This means you both go into a high level area (somewhere he can easily defeat enemies and that is many many levels above you), then you hide in a safe spot and they go and defeat enemies, giving you huge amounts of XP.

Did you modify the saves so they could work with your account? Saves are almost always tied to the account they are under, so they can't be used by other accounts. About the only game I have seen where this isn't the case is Elder Scrolls 4 Oblivion.

Also, modifying while on Xbox Live can carry the risk of banning, though the odds of you being caught for something like this is rare, as long as you don't go showing off to random people with obviously modify characters, stats, and so on.

I'm afraid the Xbox doesn't work that way. The characters you downloaded were for the PC version. The usb ports on the Xbox are made for headsets and extra storage (ex. external hard drives).

You probably have to modify the saves so they are linked to your account. A lot of Xbox games don't allow saves to work unless they are linked to your account and cannot be changed. Make sure they are linked to your account first