How to unlock this mission as Yellow Comet in Advance War 2?
That would be the Secret Lab mission, correct? You need to play the Show Stopper mission (one with Sonja). One of the properties there contains the map to unlock the secret lab mission.
If my memory is correct, it should be the second of the two properties near the western side of the fortress, as seen in this image:
Then complete the mission as usual. There is no time limit to capture the property.
This will unlock the Sea of Hope mission. Completing it will give you Neotanks for the Yellow Comet.