i want to use the Notifications in Gmusicbrowser, but i can't install Webkit in Ubuntu 12.04. Who can help me?

I'm not sure why you are talking about notifications and webkit, the notify plugin doesn't use webkit.
If you want to use the webcontext plugin, you have to install the webkit bindings for perl
As there is no package for it (as far as I know), you have to install it manually, it's a bit more difficult. I'm not sure that it's what you want, but anyway it can be useful for someone, so here are some quick instructions :

for that you need at least to install these packages (probably more that I already had) :
sudo apt-get install libmodule-install-perl libwebkitgtk-dev
download the bindings from http://search.cpan.org/dist/Gtk2-WebKit/
untar them somewhere, go there
choose a folder where the bindings will be installed, for example $HOME/libperl/
perl Makefile.PL PREFIX=$HOME/libperl/
make install

and then to use it you need to set (the perl/5.12.4 part depends on your distrib)
export PERL5LIB=$HOME/libperl/lib/perl/5.12.4/:$PERL5LIB
for example in your ~/.bashrc