If your computer isn't running Windows, one option is to run the Image Markup Tool in a virtual machine, using VMWare or Parallels. However, you may not wish to pay for a copy of Windows to run as a VM. It is possible to run the IMT under the Wine on Linux, and there is a version for OSX built using Wine Bottler (see below).
To run under Wine on Linux, you'll need to install a version of Wine which is at least 0.9.55 (released February 8, 2008). We really recommend getting the latest version; 1.0 was released in June 2008.
First, install Wine. Most Linux distributions include Wine in their repositories, but the version available for your distro may not be the most recent version; you can ensure your version is up to date by getting it from the Wine HQ. Consult this page for information on how to add the Wine repository to your sources list and install Wine. You may also want to download and install the Windows version of Firefox under Wine; the IMT offers to open files for you in a Web browser sometimes, and that functionality requires that a Web browser be available in "Windows".
Once you've installed Wine, download the IMT, and run the installer under Wine to install the program. Next, run the program and test it out. If you have any problems, you might be able to solve them by changing the "version" of Windows that Wine pretends to be. To do this, start up the Wine configuration application (you should find a link to it on the Wine menu, or somewhere on your programs menu). Go to the Applications tab, choose Add application, and browse to find the Image Markup Tool executable (which is usually c:\Program Files\HCMC.UVIC.CA\Image Markup Tool\ImgMarkupTool.exe). Select the exe file to add this application to the Wine applications list, then select it, and in the drop-down list for Windows Version, choose Windows XP, or whatever version you want to try. Click on Apply, then start the Image Markup Tool from the Wine menu. Try different versions of Windows until you get the best results.
I'm currently using the IMT myself under Wine rather than Windows, and I fixed a couple of bugs which were specific to Wine limitations in early 2010. Wine is now a supported platform for the IMT 1.8.