The mvim file is usually not writeable, change that:
sudo chmod u+w `which mvim`
Now open it for editing:
sudo vim `which mvim`
Add the following line to the top of the file, below the commented section:
tabs=true
Replace the if structure at the bottom of the file with the following:
Last step: fire up vim:
if [ "$gui" ]; then if $tabs && [[ `$binary --serverlist` = "VIM" ]]; then exec "$binary" -g $opts --remote-tab-silent ${1:+"$@"} else exec "$binary" -g $opts ${1:+"$@"} fi else exec "$binary" $opts ${1:+"$@"} fi