Eco Lab compiles out of the box on OSX using the Mac OSX Dev Tools package, which is available for free download from http://connect.apple.com
You need to make a choice whether you want to run Eco Lab as a native Aqua application, or as an X-windows application under the XFree86 server software. The only real disadvantage to using Aqua is that at the time of writing, BLT is not available for Aqua. This means in particular that the plot, histogram and display widgets will not work. Also note that the author does not have access to the console of a MacOS machine, so Aqua mode hasn't been supported for a while. If any MacOS user wishes to step up to maintaining the Aqua interface, please contact Russell Standish on hpcoder@sourceforge.net.