If when you turn on workflow activation you get the message "You must be editor of the Google Form to activate the add-on," it is probably because the spreadsheet has been shared with you, but it has a linked Form that was not shared with you.
The owner of the spreadsheet must also share the Google Form with you before you can turn Workflow Activation on.
To request access, go to Form in your spreadsheet menu. Then choose Edit form.
Now click REQUEST EDIT ACCESS
Once the owner of the Google Form has given you access, you will be able to turn Workflow Activation on.