I've had a frequent problems with that as well.

A few things to check:

* Make sure that a /files directory has been created and chmod'ed to 777 (using FTP or shell access)

* Access Control - make sure appropriate roles have the permission to upload

* Settings/Upload - expand the settings for each role and make sure that the file type you're trying to attach is included and per upload and per user quota sizes are large enough.

I have encountered situations where I was not able to upload files with the primary admin account until I created some kind of admin role, gave it necessary permissions and configured it properly in settings/upload.

Hope that helps,
Jeff