The previous software version is the one for the iPhone Mobile device. If you have already purchased it, you should be able to get it. This is the link to check.
However, keep in mind that the Slingbox Classic is no longer supported by any SlingPlayer Mobile software version.
I am having the same problem. Although the procedures in the link above say it is possible, following them does not result in the iPod/iPhone app being successfully installed on the iPad. Following the procedure, when you try to apply the changes the sync process results in the app not being added to the iPad and it reverts to unchecked in the list. Has something else changed since those procedures were written?