Select Collection

The User Interface > Main Toolbar Buttons > Select Collection

This is another split button like the Move/Copy buttons, which has a drop-down portion (the "down arrow"), and an icon.  The icon represents the collection that is currently loaded, and clicking on it will bring up the next collection in the tree (if set to do so in the User Preferences | General dialog).  Clicking on the drop-down arrow will show a menu of available collections.  Simply select a collection, and it will be loaded into the ClipList.

This is the same as choosing a collection in the Collection Tree.