Select Suggest View from the Mode Toolbar to enable Completion Support. As you add variables they will automatically be given equations and as you add connectors suggestions for equations will be shown. Unit suggestions will also appear, and units added automatically to variables when they can be determined.
To switch out of Suggest view Select either Model view or Map view from the Mode Toolbar.
This page will display automatically when selecting Online Help from the Help Menu (or by pressing F1) while in Suggest View. You can open the default help page here.