Installing Stigma2 is very easy.
First, decide whether you want the ION or the MirageOS version.
Find the one - it will be called Stigma - xxxx.8xp where xxxx is the shell of your choice. Send this to your calculator. Now, find a level pack (there are two in the 'Levels' folder). Send one of these to your calculator (you need at least one to play). You are now ready to play Stigma2!