Don't give them test scripts.
To me that invalidates the testing process to a large degree because if you're thinking up test cases your software probably handles them because you've thought of them.
The idea of good testing is that there is a level of independence in testing so you can't cater for known test cases and also the client is likely to think of scenarios that you won't, which is the whole idea.
But how do you motivate them? Well, honestly I'd be surprised if they weren't motivated. I've generally found that motivating them to comment on func specs, requirements and other preliminary documentation is a far tougher battle. By the time you get to testing, you've eliminated an important psychological hurdle in that the software is now "real".
How you handle this depends to a large extent on the nature of your relationship to the client. If you have a formal process with an agreed upon spec, you should really be saying that the client has a certain period to sign off and accept the software and inaction is implied acceptance.
If it's an internal client well then that's harder. It probably all comes down to who's driving the project? Who are the stakeholders? These are the people you need to motivate such activity.