Why do I only have the option of 'Add' on my report tab within my custom object workspace?
Custom Objects, Workspaces
The custom object workspace currently has a report control on the report tab and report controls only have the option for 'Add'. If the custom object has child object relationships, you can remove the report control from the workspace and instead add a relationship view control for the child object relationship. The relationship view control has both the 'Add New' and 'Add Existing' options.
For more information on workspace controls, refer to Including controls in a workspace.