The tension will depend on the material (knit, heavy denim, etc). See this pdf for a description of testing and setting the tension: www.ca.uky.edu/HES/fcs/FACTSHTS/CT-MMB-213.pdf
. Most of my sewing is on double knit fabric and I start with the tension on 4. (Turn the knob to the right to get a higher value and tighter tension). The stitch length depends on the material if you are basting (for test fitting or prior to installing a zipper, for example) or if you are sewing the seam. Basting uses a long stitch, about 5-6 mm (4-5 stitches per inch). Most seams for a mid-weight fabric use a stitch length of 3-3.5 mm (8-9 stitches per inch). See Table 1 here: http://www.dummies.com/how-to/content/making-machine-stitches-work-for-you.html
for more details.
To insert the cams, open the top of the machine and identify the fixed cams in the machine. If another specialty cam is installed, you may need to remove it to put in another cam. My machine (a Singer) only has space for one specialty cam at a time. The cams are labeled with the stitch that they provide and the stitch indicator faces up on the cam stack. (For my machine that means that the tall ring of the cam is up.)
A You-tube video of using the Morse 6500 is available: http://www.youtube.com/watch?v=ksfZZyqO1eE
I hope this helps.