To remap the menu key you will need to-do the following.
install prerequisites
Install package xkbset:
sudo apt-get install xkbset
define keyboard layout
Next you need to define what keyboard layout you are using. In my case gb for a british/united kingdom keyboard layout or us for United States
setxkbmap -layout gb
Note: if you run
setxkbmap -query
it will give you the output as follows - if the model and layout is your keyboard then you are using the correct layout
rules: evdev
model: pc105
layout: gb
Next define that we will be next mapping mouse keys:
xkbset m
xkbset exp =m
find Menu key-code
Map the Menu key. To do this we need the Menu key-code. In a terminal run xev, move your mouse inside the square and press the Menu key.

End Result
In my case my Menu key-code is the value 135 and this will be mapped to Mouse Button 2
xmodmap -e "keycode 135 = Pointer_Button2"
Tested as follows:
- Copy some text in a terminal - Paste action initiated
- Hover mouse over title bar - Window move to background
Putting this all together:
Ok, once we've worked out the correct keycodes, we can put this together to start from login.
First create a file in your home folder called remapkeys
gedit ~/remapkeys
now copy and paste the following
#!/bin/sh
setxkbmap -layout gb
xkbset m
xkbset exp =m
xmodmap -e "keycode 135 = Pointer_Button2"
Save. Give the file execute rights
chmod +x ~/remapkeys
Then from Dash search for Startup Applications or if using Gnome Classic - Applications - Other - Startup Applications
Add a new Startup Applications entry

The command should be:
bash -c "sleep 5 && ~/remapkeys"
The command sleeps for 5 seconds to allow your desktop to appear before running your new script - if your desktop doesnt appear within 5 seconds (maybe you are using a slow netbook?) then increase this timeout to 10 or 15 seconds.
Logout and login to test.