Before uploading from ScreenSteps to Joomla using the Movable Type Plugin you need to configure the plugin.
Choose the Extensions > Plugin Manager menu item to load the Plugin Manager screen.
Filter the list of plugins by "movabletype" (1).
Click on the XML-RPC - Movable Type API plugin to edit the settings (2).
The Plugin: [Edit] screen will appear (1). You need to set a default category in the Parameters area (2).
Select a default category (1) and click the Save button.