To make a collection available off-line once you have installed Offline mode:
You can only view bookmarked collections off-line. Check that your collection is bookmarked.
Select the Sync toggle switch to turn on synching each collection to your local computer. This stores a local version of the content to your local machine.
When you are off-line, the portal automatically recognizes that you do not have an internet connection or that you lost the connection, and opens any content you have open in Offline Mode. You can also actively select off-line mode by Selecting on My Account in the top right-hand menu and selecting the off-line option. Remember that if you select the mode, you need to change it back to on-line mode again to work connected to the portal.