How can I get Xcode to install under Lion?
When the App Store is "Installing" keep in mind this  includes "Downloading", and Xcode is usually about 4 GB. Leave it overnight or monitor your network traffic to see the progress.

Once Xcode is installed, you will need to run xcode-select to expand your path since tools like make and compilers/linkers are in /Applications/Xcode and not in your typical bash path. You can go to settings and download the command line tools to keep them if decide to delete Xcode later.