Some users have reported that after updating SignalScope to version 2.1, the update that adds iOS 4 support, their purchased upgrades are no longer available in the app.
To restore upgrades that were purchased previously, all you need to do is go to the Upgrades section of SignalScope and tap on the Restore Purchased Upgrades button. After you enter your iTunes account info, your upgrades will be restored (make sure you use the same iTunes account with which you originally purchased any upgrades).
By default the Upgrades screen (the in-app "store") is accessible in one of the tabs at the bottom of the main tab bar display of SignalScope (which is what you see when the app launches). If it isn't visible there, you can access it in the Upgrades section of the main Options screen. If/when you have purchased all available upgrades within SignalScope, the Upgrades screen will no longer be available.