Download the executable TesselMania! program.  Decompress with freeware ZipCentral.  Recent versions of Windows will do this for you automatically.

To run TesselMania! under Windows XP (or earlier verions of Windows) or Windows 7 (32 bit only), right-click on the decompressed file TSSMANIA.EXE (or on a desktop icon thereof).  Click on Properties, then click the Compatibility tab.  Under the Display options, click to select the Run in 256 Colors check box. Click OK to make the changes take effect (permanent for TesselMania! program only).

For the 64 bit version of Windows 7, you must run your computer in XP mode (see http://www.microsoft.com/windows/virtual-pc/download.aspx).  Copy then paste the entire decompressed TesselMania directory into the XP Virtual Machine screen, right click on the file TSSMANIA.EXE therein (or on a desktop icon thereof), change Display to Run in 256 Colors as described in the previous paragraph, then Disable Integration Features under Tools.  [When you set up the XP Virtual Machine for first time, the wizard will ask you for a password. You need to remember it and will have to enter it when you disable the integration features.]  Be sure to select Close under Action when you are done with the XP Virtual Machine.

BONUS:  Download TesselMania! Escher simulations.  Decompress the file as detailed above, then open any one of the 20 decompressed files with the TesselMania! program.