An old thread, but I have just come across it after trying to work out how to do this myself, so for anyone still needing it, this is how you do it.
The MVE-M300P unit should have a USB coming from it to which a USB stick can be plugged in.
In the OP's case, its either missing or hidden I'd say.
I would remove the headunit and trace the cable from the back of it that goes to the M300P box. Then you will be able to see if the USB cable is there.
Once you have that sorted, you need to do the following:
1]Register on Naviextras (the site where you get the map updates from)
2]Download and install Toolbox
3]Turn on your sat nav, and plug in the USB stick
4]Choose collect info from unit and follow the instructions on screen.
5]Now start Toolbox on your PC and plugin the stick.
6]Toolbox will now connect to the site and tell you what updates are available.
On mine I had to install a firmware update first. This involved downloading it, unzipping onto the stick and then installing it in the car by turning on, plug in stick and choose install updates.
After it restarts, start again at step 3.
Close Toolbox and reopen it, then plug in the stick again.
Now mine said I had a map update to do, so am currently downloading that.
I imagine you then copy it to the stick, plug into the car and choose add updates
Hope that helps