Under Windows (and for pak128britain experimental) you may use this: http://forum.simutrans.com/index.php?topic=6482.msg97683#msg97683
It is not an official script (it's what I use for testing), but I've never had any problems.
You must use the latest version of the script and makeobj (The makeobj there is still no final version, for now you can use this: http://forum.simutrans.com/index.php?topic=10145.msg102948#msg102948).
A few pointers for others that I figured out along the way:
1) Pass in two arguments -- one, the folder you want the compiled pak to end up in... two, the folder that contains the pak source
2) Make sure MakeObj.exe is in the folder one up from the destination. So, if you're building to \simutrans\Pak128.Britain-Ex make sure makeobj is in \simutrans
I just built the latest Pak128.Britain-Ex using this script, but the game is hanging on the "Loading map ..." step when it initially loads the demo map. Ever seen this behavior before?