In current versions of MuseScore, you can now create a pickup measure by simply deleting the unwanted beats from a measure - select the range, then Ctrl+Delete (or Tools / Remove Selected Range). You can also create unmetered measures by switching to the new Insert note input mode (see the dropdown menu next to the note input icon on the toolbar) or by using Ctrl+Shift+letter to insert notes in standard note input mode. In addition, the Measures commands have moved from the Edit menu to the Tools menu.