In order to access your mySongBook purchased tabs or the mySongBook subscription from your iOS device you need first to connect to your mySongBook account from the Guitar Pro Mobile application (not from your web browser).
1/ Open the Guitar Pro mobile application for iOS
2/ Select the Locations icon from the bottom of your screen
3/ Connect to your mySongBook account
Note that the access is almost the same from an iPad except that the tabs and the explorer are located on the left.