To: All
As always stick to the truth. Usually there are no good excuses on why you did purely on a test, except if for example you had to work on many projects (which isn't a good excuse either). I would try to approach it by showing that I have the skills that the school is looking for. Remember that the GMAT is only one component and not the most important! Work on perfecting your resume, write excellent essays; try to establish a connection at the school – show interest.
BTW, did you think of redoing the GMAT; maybe you could improve your grade by studying harder or trying a different approach.
I hope this helps,
Dror