In the title bar of the viewlet is a downward pointing triangle.   Under that is a menu which will display things you can do to the viewlet.   For this look at the Settings section.

As far as getting "foreign" viewlets onto a dashboard it depends on what friends the given application has, especially when applications are distributed amongst different application servers.   On the "Add Viewlet" pane there is a well hidden, but in plain view pulldown labeled "Select Catalog:"   On that pulldown you should see all Applications registered to the same JTS as well as any Friends.