You can select the face you want the cylinder aligned to and then press SHIFT + NumPad7Shift+Numpad 7. this will align the view to the face in a top down perspective.
Afterwards you can create a cylinder with the checkbox "align to view" selected.
In this case it is also good to know, that you can create a Custom Transformation orientation with pressing CTRL+ALT+SPACECtrl+Alt+Space while selecting the face.