Mamiya America has a PDF download of the C330 manual at http://www.mamiya.com/customerservice1.asp?id=3&id2=115
This site does not work well with anything except Internet Explorer, unfortunately.
Lens changing is straightforward. Put the camer on its back on a firm surface the first few times. Switch the side control to 'Unlock', make sure the bellows is fully retracted. Release the retaining wire. Lift out the lens.
Insert the new lens, making sure the cocking and release levers are correctly placed. If in doubt, cock the shutter first. Lock the retaining wire. Switch to 'Lock'. Adjust the parallax compensation setting on the side dial. Rotate the focus scale to the correct range.