It seems that you are having some issues with the SlingPlayer latest upgrade since your iPod will not upgrade beyond the 4.2.1 version.
The following article will provide you with the steps to follow in order to fix this issue:
Hope this helps!
This has been flagged up to Sling Media numerous times and so far nothing has happened. I really don't understand their problem, there are quite a few apps that have 2 copies in the App Store, one for OS 4.2 and one for OS 5.0. There are already 2 versions of Slingplayer in the App Store. one for the iPhone iOS 5.0 and one for iPad. All Sling need to do is post a third version which is compatible with iOS 4.2. It's not exactly rocket science.