Loss of ability to add plugins or themes


I'm running a site that was set up for multiuser, although I wound up only ever adding one other blog to it. I have recently lost the ability to add themes or plugins, as these are options that are no longer displayed. I am assuming this is a permissions issue.

The only recent changes to the site was upgrading to 3.1.3 and also deleting the one other blog, so now there is just the main site. I had given the Admin account just Subscriber powers for security reasons. The account for the blog I deleted had full Admin powers for it and also the main site. Assigning full powers to the Admin account hasn't helped.

I would be grateful for guidance in how to regain full permissions for my site.