To modify a model that is present in the source control system, you must check out the model. Checking out a model does not allow other users to modify it simultaneously.
The model has been previously checked in or added to the source control system.
In the Model Browser, in the context menu of the model, choose Check out. Look at the icon next to the model to make sure that it is not already checked out (to you or to another user).
In the Select Activity dialog box, execute one of the following steps:
Select an existing activity
Choose New
In the New Activity dialog box, enter a display name and a description of the changes you plan to make to the model.
Choose OK.
Select the activity that you have created.
Choose OK.
The model is checked out and cannot be modified by other users until you check it in.