SlickEdit's Backup History is disabled in SlickEdit Core. Eclipse maintains their own backup history which you can access using the History view (Window → Show View → Other, expand Team and click History).
You can, however, use DIFFzilla® to compare and merge with the History view, instead of using the default Eclipse compare tool.
Right-click on a file in an Eclipse explorer view (Package Explorer, Project Explorer, etc.) and select Compare With → Local History. This will activate the History view if necessary.
Select the two revisions that you want to compare, right-click, and select Compare with Each Other (DIFFzilla).
Right-click on a file in an Eclipse explorer view (Package Explorer, Project Explorer, etc.) and select Compare With → Local History. This will activate the History view if necessary.
Select the revision that you want to compare, right-click, and select Compare Current with Local (DIFFzilla).