.tun = V7 tunetool old format
.ctd = V8/V2 format, not used any more
.ctz = V8/V2 format

Move your .tun files from the desktop to My Documents\EFILive\V7.5\Bins (or similar for Win 7/8); note that this path is settable in scantool, goto Edit->Properties->Folders).

From the V7 tunetool you can save a tune file as .ctz, go File->Save Tuning File As, and select .ctz as the type.