Oracle newbie with export problem

I am trying to export my database so that I can give it to a fellow engineer. My problem is, when I right click on the database and select Export from within the OEM, I get an error that says that the preferred credentials are invalid for that node. Well, I can do pretty much everything else within the OEM using the current preferred credentials. What do I need to change? Is there a privilege I need to set? Please someone help!

There is a role of EXP_FULL_DATABASE, which you may have to grant.

Best of luck.

Don't exactly know what I did, but the problem has been resolved :)