I found a close up of the tension.
You unscrew it to loosen it and there is a slit on the side you place the tread between there
here is the pic
http://home-and-garden.webshots.com/...59188365gamZgJ
Also I looked but I am unable to find the manual for yours but here is an inline manual for mine and you may be able to figure it out from this
http://www.sil.si.edu/DigitalCollect.../NMAHTEX/0874/