You need to make sure that you have X11 fully installed on your Mac before you intall SlickEdit. (The lack of X11 is what it throwing the non-intuitive installation key error.)
If you go to the 'Finder > Applications > Utilities' > there should be a file named X11. If you do not have this then you will need to install X11 from your OS X install disc 1, by running the "Optional Installs.mpkg". During the "Install Type" phase you will need to expand the "Applications" tree and select "X11", and then continue the install as usual. Once X11 is installed, your SlickEdit license should work when installing on the Mac.