Pense-bête pour ne pas oublier avec Nemo :

Select a directory
Right click on it. Goto Open with...
Under the "Open With" tab. Select "Show other Applications".
Select Meld from the list and Add. (Don't make it the default application)
Close the dialog box.

Install Nautilus actions.

sudo apt-get install nautilus-actions

System > Nautilus Actions

Label: Meld..
Icon: /usr/share/pixmaps/meld.png
Path: /usr/bin/meld
Parameters: %M
Conditions : check “Appears if selection contains”    

Pour NEMO :

Here's a slightly different approach that works great for me. (I'm using Nemo on Ubuntu 13.10).

Select two or more text files (any extension. Say foo.php , bar.py, abc.txt)
Right click on them. Goto properties.
Under the "Open With" tab. Select "Show other Applications".
Select Meld from the list and Add. (Don't make it the default application)
Close the dialog box.

Files under : /home/qwbt2550/.local/share/nemo/actions