The problem could be that you don't have 64 bit drivers installed. Packages run in BIDS as a 32 bit process (last time I used it), which can only 'see' the 32 bit hive of the registry (containing DSN's)
Then if you run the package in 64 bit (which is generally the default under SQL Agent), the process only see the 64 bit hive of thre registry, which does not contain your DSN (or driver).
So try running the package in 32 but mode. Here's a link that may help.
None of the clues you have given actually indicate this issue but it's the first thing I thought of when I saw your error message.
If you could post more information about your configuration it would help everyone.