I neither have nor use motionbuilder, so this will be a general answer. Double-clicking on a midiout / object in a locked Max patch allows you to set an output source.
I presume that motionbuilder allows you to choose a MIDI input source, as well.
Sending MIDI from Max to motionbuilder (or any other application that accepts MIDI, for that matter) simply involves choosing a midi output destination (which is typically "from MaxMSP 1") and then making sure that your other applciation is listening to "from MaxMSP 1" as its input source.
Done man :D It was the first thing I've tried but I can't have any signal in motionbuilder. I've used noteout, midiout and other object similar but nothing. Motionbuilder shows "from MaxMSP1" and "to MaxMSP 1" but channels are as good as dead :(