To create notes on assignments and resources in MSP, you will need to go through the Usage views.
Steps -
Open a Clarity project in MSP
Click the drop-down button on View and select Resource Usage
Double-click or right-click the task and select Information. This will bring up the Assignment Information
Click on the Notes tab and add a note to the assignment.
MSP
The same steps are used to add a note on the resource, except click on the resource name instead of the task.