I downloaded the demo copy of Crossover, and there was no option to
install Internet Explorer. I couldn't locate it in any of the
extras categories.
When you select the option "Install Windows Software" from the menu, you should come up to a window that has at the top, "Select an application to install." In the larger window below that, on the left side, you should see a search box. Below that is a listing which has bold title, "Supported Applications."
Either search for "Internet Explorer 7" or scroll down the list until you come to it. That will move it into the box which will then say, "Install Internet Explorer 7 (English version)." Below that, it will tell you that it "Will download the installer... ." and below that, it will tell you that it is going to install it into a new winxp bottle named "Internet Explorer 7." If you want to, you can click on that drop-down, and you will have the opportunity to change the name to something else (whatever name you want). after you have read the information about making sure the IE7 installer's checkbox for updates is NOT checked (it should default to empty, if so, leave it that way), then you can click on the Install button at the bottom and let CrossOver do the installation, giving approval to the various dialogs that will appear.
After that process completes, you can go through the same process, but instead of selecting from the list, scroll down until you reach the entry under "Unsupported Applications" which says, "Other Application." Select that option. Then on the line below the window, select your installation file. On the bar below that, select the bottle created in the first step as the location to which it should install your program. Click on "Install" again, and the program should then be installed.
As I said, not having a copy of the program, I can't verify that this proceedure will work, but it should. However, I am basing my post on Linux. It is possible that the Mac version is somewhat different, but there is no way for me to check. Please let me know whether it is successful for you or not.