In the Presentations preference pane, uncheck everything and restart OpenLP. This should get the Presentation tab to show.
I'm not sure what causes this, but my theory is that one or more of the presentations in the database is probably corrupted or missing, and this causes OpenLP to not display the Presentations tab. In my case, I only had a few presentations in there, so I just deleted all of them and then re-enabled the appropriate options in the Presentations preference pane.