The reason you arent seeing the CreatedBy field in the Toolbox on your CRM_NewActivity screen is because its actually already on the screen by default. It is Located under the Additional Info section of the screen. By default, this field is Read-Only, and will be populated with the Username of the Currently Logged in User once the Activity has been saved. If you create a new activity, Navigate back to the Activities list, and then open and edit your Activity, You will find that the Created By field will be populated.
If you were wanting to manually set who the Activity is Assigned to, You can set turn off the "Read only" option of this field by customizing this screen, Navigating to the Created By field and clicking on Edit.
On Step 1 of 6: Dropdown Field Properties for field CreatedBy you will see an Advanced Options (click to show) button. Click on this and Uncheck the box beside Read Only: do not allow editing
When you publish this screen, you will then find that you are able to select from a dropdown list of your current users.