You should see your menu icon in the upper right corner...most likely 3 vertical dots. Tap that. In the menu that appears tap on the 3rd item, "Save to Offline List".
To remove a cache from a list, go to the list, pull up the cache, tap the menu icon, and the 4th item is "Remove from Offline List".
Thanks for the response but, when I tap on the triple dots (on the summary page of the cache that I want to add to an offline list which is not already in one of my Offline lists), my third option is "Add To Favourites". This option doesn't work if I just want to add a new cache to my Offline List.
I have just checked the summary page for a cache which is already in one of my offline lists and it has the menu item just as you have described! What about if I am viewing/adding caches that I do not already have in Offline Lists?