You want to enter actual costs, but you find out that you can’t do that. That’s because Project automatically calculates actual costs based on the actual work accumulated or materials consumed on tasks.
You can enter actual costs on assigned tasks that have been completed (remaining work is zero) in any view that has a cost table applied.
To enter or edit actual costs manually when work is still in progress, turn off the automatic calculation of actual costs.
Choose File > Options > Schedule.
(In Project 2007, choose Tools > Options > Calculation.)
Under Calculation options for this project, clear the Actual costs are always calculated by Project box.