el236 -- I don't know where to get more info off hand, but I can explain it briefly if you like.
Suppose you have a CMYK job and you want to use the color red... you just create a new CMYK color, mix magenta and yellow, and make sure Spot Color is unchecked. Pretty basic stuff.
Now suppose you have a different job that uses, for example, 2 pantone inks instead. How do you create a color that uses 10% of one and 60% of the other? With Multi Ink mode. Just create a new color using the Multi Ink mode, select each color in turn and click on the 'shade' drop down list to pick 10% and 60% respectively.
You'll probably notice that the four process colors are always available too. Mixing them together in multi ink mode is exactly the same as mixing them using the CMYK model. The only advantage is that if you want to mix a spot color with a process color, you can do that here. If you just mix process colors, then you'll get a normal process color.
If you want to turn a multi ink color into a spot color, then make the color you want, change the model to CMYK, and check the 'spot color' check box. To be honest, if you just want to create a spot color out of process colors, you're better off using the CMYK model and checking 'spot color'. Does this answer your question techleo? I've been working for 24 hours straight so forgive me if I've misinterpreted your problem....and sorry about the long essay! Let me know if anything needs clarifying.