As suggested by Fiendish, it is possible to run MUSHclient under OS/X without needing to use an emulation program like VMware, or Parallels.
Requirements:
Simply download and install Wine, currently the latest version is available here:
http://darwine.good-day.net/Wine_1.2.2.dmg
After you open that file you should see something like this:

Drag the Wine application to your Applications folder.
Then you should be able to double-click the MUSHclient installer to have it installed into a "Wine Files" folder in your Home directory.
After installation, you should be able to run MUSHclient in the usual way. Here is a screenshot showing my local SmaugFuss server, including the mapper:

You probably want to get a better font in the Output window, like "Bitstream Vera Sans Mono" 10 point.
Requirements:
- A Mac with an Intel CPU (All new Macs use Intel CPUs). Older PPC or 68k Macs cannot use Wine. Sorry.
- X11 (this is already installed by default on Snow Leopard and Leopard. If you are running Tiger, you can install X11 from your Tiger installation DVD using the "Optional Installs.mpkg" file.)
Installation
Simply download and install Wine, currently the latest version is available here:
http://darwine.good-day.net/Wine_1.2.2.dmg
After you open that file you should see something like this:

Drag the Wine application to your Applications folder.
Then you should be able to double-click the MUSHclient installer to have it installed into a "Wine Files" folder in your Home directory.
After installation, you should be able to run MUSHclient in the usual way. Here is a screenshot showing my local SmaugFuss server, including the mapper:

You probably want to get a better font in the Output window, like "Bitstream Vera Sans Mono" 10 point.