I was following along doing the mission for Mr. Maxis and accidentally clicked on the X to cancel his mission. Is there anything I can do to get this mission (and presumably this chain of missions) back?
Nope, but the mission will most likely pop up again in the future it it was cancelled prior. Although, as stated above many of the missions are more like tutorials than actual missions. There are also things like the DR. VU or MaxisMan that are more indirect missions