It is extremely disappointing that Texas Instruments provides no support for OS X whatsoever, particularly in light of the fact that CCS, being Java-based, could easily be made to run under OS X. Most of the Windows-based development tools will run in a virtual machine under OS X, but that does not count as support -- at least not in my book. Those interested in a native IDE for OS X should investigate Rowley Associates CrossWorks.
Fortunately, the GNU GCC toolchain has support for the MSP430 and it runs quite well under OS X. Getting it to compile can be tricky if you are not familiar with building software. TI has a wiki page with some instructions. You can also find some helpful information in this thread in the e2e community. TI also provides a Virtual COM Port Driver, but reports are that this is completely broken.
The best way to get mspgcc4, mspdebug, and the proper USB driver installed quickly and easily is to install Javi's OS X Launchpad bundle. While I have not used this bundle personally, reports are that it works quite well.
Mspgcc4 does not provide support for some of the latest chips (such as the MSP430G2553) but there exists a new development fork known as "uniarch" which will be supporting them in the very near future. If you want to live on the bleeding edge, follow these instructions on building uniarch for OS X.
This section contains information on building the GNU Toolchain under OS X. Usage of these tools can be found in the GNU Toolchain section.