I would contact the nearest blood donation center to discuss. I am sure they would be more than glad to talk with you!
I know that drug is not specifically listed as an exclusion but they would know. There are plenty of other things that can unqualify someone as a plasma donor.
AABB Donor History Questionnaire Documents
Here is a thread on the topic but that is for whole blood; plasma donations sometimes have more specific exclusions.
Enbrel and Donating Blood (http://talkpsoriasis.org/showthread.php?23264-Enbrel-and-Donating-Blood - broken link)
It is admirable for you to want to donate. More people who can should!