Assign resources to activities
Choose Project, Activities.
Select the activity to which you want to assign a resource.
Display Activity Details, then click the Resources tab.
Click Add Resource.
Select the resource you want to assign.
If resource security is enabled you can only select resources you have access to.
Click the Assign button, then click the Close button.
For more information please see the video below, it also include cost loading the schedule in Primavera P6.