slplus will always be undefined if you are getting the "Store Locator Plus Script not loaded properly". There is a self-check built-in to make sure the proper javascript is running an initialized. If not, you get that error message.
You can try turning Force Load JavaScript off. It should be on by default. However if that is not working I suspect larger issues, possibly with the theme. If a theme does not employ WordPress 3.3+ compliant script management via proper enqueue script calls and proper page footer processing it is going to be ongoing battles to get SLP to work in that environment.
I suggest trying first with a base theme like Twenty Ten/Eleven/Twelve and see what happens. Sort out any issues there first as those 3 are very well tested. If those themes have problems there is a good chance the PHP configuration and/or server security settings are getting in the way.
