Real annoying Mission Editor Problem!
 

ERROR: Player's unit _Type VIIC#1 enters the game after mission start.

The earliest non-AI U-boat I can use is 30/07/1940.

The latest date I can start a mission at is 1/1/1940. I've tried changing the mission start date to the same date as this U-boat, but it always jumps back to 1/1/40.

Hence the above error. Does anyone know what my be causing this?

manually open the .mis file with windows notebook and edit the start date that way then save and try the mission. i dont know if it will be called "player unit" (its been a while) so look for the boat type you chose

sometimes there are things mission editor wont do that can be done manually

Thanks!

Although my original question was asking how to add a player unit, I found this out from a Mission editor guide.
The problem then was I could not validate the mission becasue of the error stated - so I edited this question but could not change the title.

However, what you suggested has worked.

Thanks!