Ouch - really struggling to remember my manual-focus Macro lessons at college 20 years ago.
I seem to recall that you multiply the metered shutter speed by the extension ring number - for a 1.5x extension ring, x the shutter speed by the same factor (a 1/30 sec exposure becomes 1/15 and so on...).
When using TTL metering, it was supposed to correct automatically, but never did, so we all used hand-held meters and calculated accordingly.
How to here:
http://www.kodak.com/global/en/consu...20.shtml#50849