Scenario:
Web TimeSheet user is unable to add activities even though the permission assigned to the user has View and Add Activities option enabled. The Activities link under Timesheet Setup in the left menu is missing.

Cause:
This issue is usually caused when the Activities section of the users permission does not have the Edit option enabled.

Resolution:
Make sure that the Edit option is enabled apart from View and Add options, under the Activities sub-section of the Administration section in the users permission.