Verify you are logged in with your AltiumLive account and you are using a valid license. Then ensure you have a schematic sheet or a PCB document opened on the background, and no other windows like Draftsman, Outputjobs, etc... Sometimes it is necessary to close and reopen the MPS panel after making changes to your login or license status.
Also try Clearing the cache under Preferences ► Data Management ► Servers ► expand the Known Servers section ► click on the Clear Cache control at the right side of the line at the bottom that says "Automatically connect Altium Content Vault".
If a part doesn't have a model, you can't use most of the MPS panel right-click options. You can however create a new component in your library and then while editing it, pull up the MPS panel where you can find the desired part and use the right-click menu to Add Supplier Link [And Parameters]. Component entries listed with an associated [green IC chip] symbol are matched to a corresponding Managed Component in the Altium Content Vault, have associated model data, and are therefore ready to be placed in a design and can be acquired. To restrict the listing to those components that have associated model data, select the Yes option for the "Has Model" parameter near the bottom of the Filters pane.