Hmm, I can imagine that you might have tried to install Heretic II once, and then it failed because of ixemul.library not was installed. The first time you run the installer, the installer creates a file in ENVARC:Heretic called Installation where the installation path is stored (for instance SYS:Heretic). The second time you run the installer it can't be run correctly, because the Installation file is already written, and the path it is pointing to has been deleted.
To solve the problem, simply delete the Heretic drawer in the ENVARC directory, just write "Delete all ENVARC:Heretic", reboot and start the installer again.