Let me clarify this questions that you have before purchasing a new Slingbox PRO-HD.
Can i use my wireless adapter for xbox and plug it into a slingbox for wireless internet?
The Slingbox does not have wireless capability, neither will work with the USB Wireless Adapter for the Xbox. Instead you can use a wireless bridge or SlingLinks if the Slingbox can't be hardwired to the router.
Also, if i pay the $29.99 for the mobile app does this cover both my ipad and iphone or do i have to pay for each of them?
If you purchase the SlingPlayer Mobile for iPhone or iTouch, you can use that application in Compatibility Mode on the iPad, but the experience will not be optimal. SlingPlayer Mobile for iPad is a completely new application designed for the iPad, therefore you will need to purchase it separately.
This link will give you more information about this:
I hope this helps!