Creating an Activity for a Unit is much like creating an Activity for a Building.
Go to the Properties menu and select Buildings.
Select the Building in which the Unit that you want to create the Activity for resides in.
Select the Activities tab.
Here you will see all the Activities for this Building and also for all the Units in this Building. Click on Add Activity to create a new one.
Fill in the details in the same way as for a Building. The difference here is that you also need to select a specific Unit.
Pick the Unit you wish to associate the Activity with.
Your Unit will have been added to the Activity.
Who's Responsible and Schedule
Fill in the remaining information such as Assignee, Due Date, etc. as when creating for a Building, and then save.