I would check around, it may be that Speedgrade doesn't support .MTS files. Since it's a recent acquisition of Adobe's, it may not be as robust when it comes to different file types.
You may just need to use the "Send to Speedgrade" command to render out DPX frames from Premiere.
Angelo Lorenzo
Fallen Empire - Digital Production Services
RED transcoding, on-set DIT, and RED Epic rental services.