the HotPot module is initially disabled in Moodle 1.5. You can enable it on the "Adminstration -> Configuration -> Modules" page.
On the "Adminstration -> Configuration -> Modules" page is a list of modules installed on your Moodle site. Next to each module name you will see an eye icon. A closed eye means the module is hidden (=disabled), and an open eye means the module is visible (=enabled). From what you say, I expect that on your site the HotPot module has a closed eye icon. If you click on this closed eye icon, it will change to an open eye, and thereafter you should have "Hot Potatoes Quiz" available as an activity type on your courses.
At least, that's my understanding
all the best