You should be able to update in either of two ways:

As Clay suggests, you can download

http://www.minorplanetcenter.org/iau/Ephemerides/Comets/Soft02Cmt.txt

and then use 'add MPC comets/asteroids', clicking on the line near
the bottom of the resulting page.

_Or_, you can grab the file Hartwig has provided,

http://www.gva-hamburg.de/guide8/guidetest.zip

and unZIP that in your Guide folder. After doing that, you can
again use 'add MPC comets/asteroids', and then just use 'Click to
download updated comet data and add it to Guide'.

This will automatically download the same 'Soft02Cmt.txt' file
mentioned above, then merge it with the comets Guide already knows
about. The end result should be the same, no matter which way
you do it.

I'm in the last stages of getting an update together that will
include, among other things, the same fix Hartwig has provided
in 'guidetest.zip'. I hope to post it to the Web site later today.

-- Bill