To quickly generate new packages with your patch applied, run the following in a gnome-terminal:
sudo apt-get build-dep indicator-applet
sudo apt-get install devscripts
apt-get source indicator-applet
cp /path/to/patch/indicator.patch .
patch --dry-run -p1 < indicator.patch
Continue only if that succeeds without errors.
patch -p1 < indicator.patch
debuild -us -uc
Now wait for the thing to compile and if everything worked, you'll get some debs ready for install:
sudo dpkg -i *.deb # I'm lazy, just choose the debs that are already installed
You'd need to do this every time the indicator-applet gets updated (but then I don't know how often such updates happen in stable releases).