How to view the internal signals of module in ModelSim using the testbench?
There is a chance, that you didn't compile all the updates in your code.
Here is how it should look:

Basically, you can add any signal in your project to a waveform to simulate. After creating Simulation Configuration you double click on it in a Project tab, which should get you to the sim tab. You will see the hierarchy of your project there. By clicking on the name of the entity under test, you will be able to see its internal signals in the Objects tab. Then you just right click the desired signals and choose Add Wave.