To view, the OS maps go to Maps (top menu) and select the map set you have on your GPS unit. 

You may have to zoom in on the maps for the OS maps to show.

There is a video in the online resource taking you through this -

Go to online resource (login), then Courses > Garmin BaseCamp > Garmin BaseCamp - mac users > Unit 3 - Viewing your maps that are on your GPS in BaseCamp and an overview of the map page tools.

I hope this helps.