To manually install Protective Markings on Outlook for a Mac you first need an addin manifest file. Preemptive will either provide you with a demonstration manifest file, or one customised for your organisation.
Once you have the file, follow the procedures below to deploy the addin.
On the toolbar click on the Get Add-ins button.
In the left hand navigator click on "My Add-ins"
Click on Add a custom add-in, Add from file.
Select the manifest file provided, click Open
Now when you send an email you should be prompted to select a Protective Marking.