Firstly, please check through the emails you have been sent previously relating to OVERDRIVE.
There'll be one, in particular, marked "do not delete, store, save, flag" etc. or words to that affect.
This is the one that contains more information on your member benefits, as well as a link to the private/secret members portal.
Failing that, one of the many other emails you've been sent about OVERDRIVE will include the link.
If you literally cannot find the link anywhere, despite the myriad emails, then raise a support ticket and we'll endeavour to get back to as swiftly as we can.