I am not a 'nix nerd so this took me a little time to sort out. Anyhow I have got it working, and in case this helps anyone else, this is how to do it. Just launch the Terminal and at the prompt type
exec ~/Applications/Blockland/Blockland.app/Contents/MacOS/Blockland -dedicated -mission base/data/missions/bedroom.mis
and hit return. The dedicated server should then start.
This assumes that you have the Blockland folder in the Applications folder in your home directory (the logical place for it). Edit the command to reflect where you have put Blockland, and the map you wish to serve. Note also that the server quits when you close the Terminal window. You don't need an extra computer to run a dedicated server, you can do so by using another user account on the same mac.
I have one further question. I tried running a server using some of GSF's San Andreas maps. However each time I tried to join, (using the other account of course) I got the same error message "invalid packet : 12" and was thrown off. What does this mean? Another 3rd party map "The Valley" exhibits no such problematic behaviour.