To invoke it:
Application.CommandBars.ExecuteMso ("ReviewCompareAndMerge")
You can find the name of other commands to use like this by going to the ribbon customize dialog box, finding the command you're after, then hovering the cursor over it. The tool tip that appears will show you the magic bit of text to use (it's the last part, in parentheses).
Ah. Or:
ActivePresentation.Merge "c:\temp\test.pptx"