There is two way you can play your edited mission in Driver. The first is by using a mission name that you will be able to select from within Driver and the second is by adding them in the Undercover Mission Tree.
Using known mission number from Driver.
The easiest way to play your edited missions is to save your missions with a mission number actually used by Driver, then playing that mission number from Driver. You can see the mission number and where they are used in Driver with the Mission Selection window. Look at the 'Driving Game' or 'Take a Ride' folder. For sample, let's say i whish to play my mission from the 'Driving game' / 'Pursuit' / 'Miami 1' menu in Driver.
Adding my mission in the Undercover Missions.
This is probably the best way to do it since you can actually create new mission number without using or modifying the actual content of Driver Missions. Best of all, you can actually add more than one missions that you will be able to play one after another, just like the Undercover mission.
Putting back the original Undercover missions
Later, if you want to get back the original Undercover missions of Driver, select the 'Restore to Default' option in the Mission Menu. You will probably want to save your current 'Mission Tree' with the File Save As option from the File Menu before Restoring to default, so that you will be able to recall them.
Recall your Mission Tree
Lets say you have saved your mission tree as 'mymission.dml'. Load your 'mymission.dml' from the File Menu (you can select the DML file type) and then save your mission as mladder.dml.