I've never messed with marvel mystery oil, but I've used seafoam in both cars and bikes. Great stuff!
In cars they suggest to divide the can into thirds. One goes in the gas tank, one goes in the crankcase with your oil, and the other you either dump directly into the carbs or, if it's fuel injected, disconnect one of the vacuum hoses from the intake, submerge the hose in the cup of seafoam, and crank the engine. The engine will suck it up and not start. Reconnect that host, close up your oil cap and your fuel cap and let it sit for a few minutes. Then get in, crank her up, and give it gas until it finally catches and fires up. Smoke will be billowing out for a good while, and that is just your carbon deposits burning away.
In your bike It's a little harder to do it the same way. You also have to keep in mind that since your bike has significantly smaller quantities of everything in comparison to a car, you must use less seafoam. On the can it tells you exactly what the ratio is of oz. of seafoam to gallons of gas as well as oz. to qts of oil. You can try to use a syringe, turkey baster, etc to put some of the seafoam directly into the carbs through their throats, but I wouldn't bother. Just add the proper amount the gasoline and it should work fine. That's what I've done.
It's about $8 at autozone depending on when you get it, and it has instructions on the can. Good for just about any engine you can think of. IMO it's money well spent, as it seems to do a good job cleaning.