When you install Amplitube it should let you choose if and what directory the VST is installed. Somewhere like C:\Program Files\VSTPlugins for VST2 or C:\Program Files\Common Files\VST3 for VST3 (you're locations might be different.) Find where it was installed, easiest way is to start installation again to see where it wants to install it and cancel out. Then in Studio One, choose Studio One > Options > Locations > VST Locations and Add the location.