Sorry, didn't see this earlier. By "Open File" I meant the "Open File" button in SIR. Click on it, and navigate to the impulse file (which are .wav files) and load. It should load in SIR and you should see its waveform there.
So step by step:
1. In Cubase's arrange window right-click in the track name area and select "Create FX Channel"... select Stereo and OK.
2. Open the Mixer. In the mixer click on the little "e" button for the FX channel that you created to open up the channel edit window.
3. In the window, click on one of the empty insert slots and select SIR. TIP: With reverbs and such, I normally insert them in the second insert slot, just in case I need to put and EQ and the like before it.
4. The SIR window will open.
5. In the SIR window click "Open File" button, navigate to the impulse, hit OK to load the impulse.
6. Click on the little "e" button for the track you want to apply the reverb to.
7. On the right, slots (the send slots), click on the top one and select the FX channel SIR is on.
8. You can adjust the send amount by using the green slider directly under that send slot.