Auto-Renew option not displaying membership contribution page
I've found that this happens if Pay Later is selected in the contribution - and this is not the behaviour I'd expect. If I turn off Pay Later, the auto-renew checkbox is ticked - but also readonly, which is also not what I've expect for an optional auto-renew. I'm on 4.6.14 and Drupal. Anyway: this might get you going.

Here's my similar question