Sometimes the Add New Content, Edit Current Content or Remove Current Content buttons go missing from the Page Content Tab.

This only happens if you created or added bad HTML into one of the assets on your page.

Usually Firefox ignores the problem but other browsers, especially Internet Explorer, will suffer by not showing the edit buttons.

Should this happen, you should probably load up in FireFox and try to correct the problem.  If you can't do that then you should delete the troubled asset from here: Website Controls > Delete Content (there is no need to delete the page, just the asset on the page). You can then try to re-create the asset properly as the buttons should re-appear.