The new levels starts after the original 5. If you want to jump right in you can get the level passwords here.
I tried to make them a bit harder, I just hope I haven't made it too hard.
If you already have v 1.1 of the original downloaded you can download the Squeaking of the Dead Upgrade installer. Just locate the Grimp folder and hit install and it should upgrade it.
Otherwise download Squeaking of the Dead Complete it contains everything from both games.