You can focus at infinity no matter where the mirror is. Turning the focus knob changes the point of focus (how far beyond the back of the scope the image comes to focus). Try focusing the Moon on a sheet of paper when you have no eyepiece. There is a point where the Moon will come to focus. Turning the focus knob will change where you have to hold the paper but you will always be able to find a place to focus the Moon on some paper.
Now let's assume you have an eyepiece or CCD at some fixed point. On the back of my focus knob is a marking that indicates that you get to infinity by turning the knob left. If you turn it too far "beyond infinity" then it won't focus on anything. If you turn it to the right of infinity then it will focus on objects closer in.
The infinity position of the mirror depends on how far back you have the eyepiece. If you have a diagonal then it is further back than if you don't.
I would just turn the knob clockwise until you can't go any further. This will move the mirror all the way to the back. Then turn the knob left while looking at something bright (a tree in the daytime, the Moon at night) until it comes into focus. You may have to turn the knob 20, 30, more? times to get focus.