File > Send to Mudbox

NoteSend to Mudbox is only available in the Maya File menu if you have matching versions of Maya and MotionBuilder. (Maya 2013 and Mudbox 2013 are matching versions.)

Send as New Scene: Opens a new scene in Mudbox, and sends the selected objects from Maya.

This option launches Mudbox if it is not already running. If running, Mudbox prompts you to save to the current scene or cancel. Selecting Cancel stops the send operation

If you have selected objects in the Maya scene, only those objects are sent; otherwise, the entire Maya scene is sent.
Update Current Scene: Available only if Mudbox is running. Updates the Mudbox scene by sending the current Maya selection. If no objects are selected, this option sends the same selection that was previously sent.
Add to Current Scene: Available only if Mudbox is running. Sends the current selection to Mudbox. Any selected objects that have already been sent are duplicated in the Mudbox scene.
Select Previously Sent Objects: Selects objects that were previously sent to Mudbox. If you have previously sent multiple objects at the same time, this option selects all of those objects. However, if you have sent multiple objects separately, this option selects only the most recently sent object.

This option is a convenient way to see which objects are already being shared, and to choose which objects to add to the selection.