You can retrieve a link to registration by navigating to Setup->Registration->Registration Setup. A link to registration will be found at the bottom of the page.

New Layout:

Old Layout:

Keep in mind that each league and season has its own individual registration link. The link given at the bottom of the Registration Setup page is for the league and season that you are currently in on the admin panel (check the league dropdown and season dropdown at the top of the admin panel). Make sure you are in the correct league and season for the registration that you would like to link to.

Alternatively, the registration module also provides a link to registration. The registration module always links to registration for the current active season.