The steps I used are as follows (do at your own risk):

1. first create a suitable directory for the map files.  (I chose \Program
Files\Garmin\Mapsource\Maps).
2. Make a subfolder in your newly created map folder for each CD.  (I
created  MGEast folder for Metroguide East).
3. Copy all the CD *.img files into the new folder for that CD. Note: you
should create a seperate MGRegion directory
    for  the metroguide region files even though they share the same CD
with MGEast
4. Next retarget Mapsource by starting up REGEDIT.EXE. (It is good
practice to export a copy of the
    registry using the Export function before making any changes).
5. With the Find function, Search for the "Garmin" folder.  Go to
    Garmin->MapSource->Products.
6. You will see numbered folders for Each Garmin CD installed.
7. Change the "Loc" field from your CD drive to the folder that goes with
that product. The "Bmap" field will
    clue you to the proper CD.
    " Bmap=D:\Program Files\Garmin\MapSource\Topo100USE.img"  Pattern
corresponds to
       the US E  or Topo US East CD.  In my system I set "Loc" to my
"TopoEast" folder:
       "Loc=D:\Program Files\Garmin\Mapsource\Maps\TopoEast"   directory.
8. Go through the remaining numbered folders and make the appropriate
changes.