When an author is created in the system, there are a number of "permissions" that the author can be given, one of which is to upload files. It looks like you're missing quite a few permissions there.
If your username has been given permission to Add/Edit Weblog Authors (this is a link you should see when you first log in to MT, on the right hand side, above "Edit Your Profile"), you should be able to add these permissions for yourself. But, if the person who set up your account did not give you sufficient permission to do this, you will need to have them login with the initial author account, and adjust the permission for you.
Or you could use
MT Medic to adjust your permissions.

The full list of permissions is this:
Post
Upload File
Edit All Posts
Edit Templates
Edit Authors & Permissions
Configure Weblog
Rebuild Files
Send Notifications
Edit Categories
Edit Address Book
Hope that helps.