Just set up the trial of the Flash Actionscript IDE FDT. There’s a few things to keep in mind when installing it on Mac OS X, however:
1) You need to download the standalone Flex SDK, and point to that in the Core Libraries Settings (Preferences > FDT > Core Libraries > AS3 Core Libraries). If you’re running the standalone Flex Builder and try to point to the SDKs in that install it won’t work. Make sure that there’s no spaces in the path to the SDK, either, otherwise FDT won’t be able to find it. I put mine in ~/Documents/sdks/flex_sdk_3/
2) You need to copy the playerglobal.swc file to a location without spaces as well, as suggested here. I put mine in ~/Documents/fdt/playerglobal.swc.
Those were the two big differences from the instructions in the Basic AS3 tutorial (Help > FDT User Guide > Getting Started > Basic AS3 Tutorial).
I’ll be putting FDT through its paces, comparing it to TextMate of course. I have a feeling for big projects it’ll be pretty useful, since it has the project-level hinting/completion that TextMate lacks.