The Check Out model command is available from the Model cascading submenu, which is a part of the ProjectWise cascading submenu:

Selecting the Check Out command opens the Select Model to Check Out dialog. The dialog lists only models in the currently open project which are still available for check out.

Select a required model in the list and click Select to check it out.

Successfully checking out a model results PlantWise prompting whether the checked out model should be opened.