From what I read, it sounds like your lens may not be compatible with the Canon 2X converter.
http://www.usa.canon.com/cusa/consumer/products/cameras/ef_lens_lineup/extender_ef_2x_iii#Specifications
Read the "NOTE" near the bottom of the page.
..
Kia Ora. I have a Canon 600D DSLR with a canon 18-200 lens on it. Can anyone tell me what the pros and cons are for either getting a 400ml lens (or bigger) as opposed to getting a 2 x converter which is considerably cheaper.

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic