You cant see the similar content block, because the similar content block is already on that page in the top right. Its not showing anyhing, because there is nothing it deems to be similar
If you go to System->Settings->Login & Registration, you can change where that button points to, by changing the 'Allow new registrations?' setting to point to an external site
You would need to change this within the stream settings for whatever stream you are looking at. If you take a look at the following guide, you will see how do amend these to your liking