using SignalTap for internal RAM

You can't. You can use SignalTap to monitor the signals connected to the RAM block and see what your system is doing but you can't access the RAM contents yourself with SignalTap.

You may be interested in another tool in Quartus called the In-System Memory Content Editor. It can let you access a RAM block if you set the correct option before compiling the project. You can only use it on single port RAMs though (as the editor will use the second port to access the RAM).