As posted by Ken & Mark, it's tough to see it on the CCD. If you can use a EP then that would be ideal as you get instant feedback and dont need to worry with the CCD. I have used the method you linked to for my reflector and had no issues seing the disks using a 17mm or 13mm EP. However it isn't important that you see the rings; just the shape of the out of focus star and position of the secondary in the middle of the disk.
The whole point really is to see how the disk (Defocused star) is 'pinched' and how the shadow of the secondary (the dot in the middle) is centered. If the disk isnt circular then the optics are either pinched (Too tight on one side) and is being distorted or the optics itself isnt round. In your case if you see this then it would be the first since you have good optics. Furthermore the dot in the middle of the disk should be exactly in the center. If it isnt then it's a very good indicator that your optics arnt aligned and is a good way to adjust your optics to get them bang on. This is also a similar approach I use on my RC8, and gets me very close.
Dont worry too much about not seeing the actual rings in the disk, they would just be concentric with the outline of the defocused star and thus you can just use the shape of the disk. The rings wouldnt really get used nor are critical to get the collimation because of this fact
