Frankly, I'm not sure if osx 10.5.8 is any good for coding against forge. It's old now and the installation process for forge means that you may have issues getting python and java set up properly in order to run the forge installer. There are posts elsewhere that cover setting up java and getting the right version of python.
Trying to set up forge failed for me with issues around the script failing at the decompile stage. The problems centred around the astyle program used by the installer. To solve the issues I installed a new version of astyle, and changed some options the forge's configuration file/s for astyle.
The forge setup package comes with astyle - you can find the osx version of it on your system here: forge/mcp/runtime/bin/astyle-osx
Download astyle from sourceforge and follow the installation instructions on the site to 'make' it on your own machine.
Once done building your new version of astyle, rename it to astyle-osx and replace the original in forge/mcp/runtime/bin/ with your new one.
Once I re-ran forge's install.sh it produced another issue, still failing to decompile.
The mcp.log file said that one of the options for astyle (from forge's config files) was invalid.
The option in question was max-instatement-indent=2
I checked the online documentation for astyle, and it seems that the option should be valid, but running the command astyle-osx -h (help) (with the new version of astyle on my machine) gave the info that the minimum this option can be set to is actually 40.
Looking through the forge directories, I found 2 instances of config files for astyle (called astyle.cfg) - one in forge/fml/conf/ and one in forge/mcp/conf/.
I changed the value for the option to 40 (i.e. max-instatement-indent=40 instead of max-instatement-indent=2), and re-ran forge's install.sh script ; this time it ran through to completion.
This might just have been be a problem on my particular setup, but if not, I hope this solution helps someone else.
(This was originally a response to another post, it's since been rewritten to make more sense standing alone at the top of this thread)