I've got an application with the above mentioned technologies, it seems to work great using a mock DAO to display my objects on the JSPs.
I've created a form pre-populated with some mock data, and I'm trying to persist this, but this is where I'm coming across a nullpointer and I have absolutely no idea why, I've followed lots of tutorials and have spent the whole day searching for an answer
In my MockDAO I return a list of my objects which I use to display on my JSPs, what I've done is setup a form pre-populated with one of those objects, then I'm clicking a "save" button which should (via the service) invoke the .persist() on that object.
I've got all my hibernate / HSQLDB jars etc on the classpath, but I haven't explicitly done anything with HSQLDB as I wasn't aware I need to
Stripersist should handle creating the entitymanager for you. There is an interceptor in the library that does this and opens and commits sessions for each page request. You'd probably be better off asking Stripersist specific questions on the Stripes mailing list since that is where the project author(s) hangs out.
Now, on for a very opinionated piece of information. You're already using Spring and Hibernate. I love Stripes. But considering your stack, I feel you'd be better off using Grails if you can. It uses Spring and Hibernate as well. It just makes things TONS easier.