The latest matchmaker has a major update to the map pack system with a complete rewrite of the back end and a new method of installation. Map packs are now installed into a separate directory leaving the original Populous files untouched. This should cut down on the permission and corruption issues people have seen that require a full reinstall of Populous to fix.
There is a little bit of setup required to copy the files that is done the first run of the matchmaker. If there is ever a problem with the game not starting or issues with levels, just type !restore to recopy the files.
Another new feature in the update is testing of maps not in a map pack. Often map creators found that it was difficult to try out their map in a multiplayer setting to work out balance and game play issues, so this should help with that.
In the matchmaker, enable map testing on the Populous tab in Options (all testers must do this). There will then be a new "map pack" available labeled "* Test Map" at the top of the map pack list. Click that and pick the map to play on the list for install. Only the host needs to select the map.