Neil, I think it's all the blather about unlock codes that have you bothered. With the topo maps you should be able to cheerfully ignore anything to do with unlock codes. I'm not aware that Garmin has ever sold any topo products that required unlock codes. As far as I know all their topo products are unlocked.
You should just be able to install the maps from the CDs, run Mapsource and follow the instructions for selecting and transferring map data to Mobile PC.
Worst case is that you probably need to update Mapsource to a newer version. The version you have might not recognize Mobile PC, which is a pretty new product. Since you already have Mapsource installed, you can just go onto Garmin's site and download and install the latest.
Garmin: MapSource Updates & Downloads
Don't remove the old one. The Mapsource updates need to see that an older version is installed or they won't let you install the update. They just install nicely right over the old version.
Once you have Mapsource updated you can follow the instruction in that other thread, keeping in mind that the first time you try to transfer to Mobile PC you will probably need to have it running for Mapsource to find it. Once Mapsource has found and transferred to Mobile PC once, you should not have to have Mobile PC running to transfer to it after that.
If you still have a problem, can you please give details about what steps you went through and exactly what results you got, including any messages?
...ken...