Believe it or not, it is possible for a B757 to have an MCDU fitted, if it has a Pegasus FMC! You'll probably never hear it get called that though.
I don't know of any budget FMC programs, only the extremely costly CBT stuff.
However, if its just a basic guide you're after, then Bill Bulfer's 'Big Boeing FMC User's Guide' is ideal. It covers the 74, 75, 76 and 777, as there is a large degree of commonality. You should be able to get it from any aviation book shop.
If you're converting from a 'bus to 757 it will be a piece of cake! The Boeing philosophy is much, much easier and more intuitive than Airbus.
R.