1. Download the tar.gz format of Hydromad package
2. You have to download the Xcode 5 from Appstore. You have to register first as developer, as it was made for developer. It should be properly installed in you Mac. But you don't have to run it.
3. Run the "terminal' window. The icon should be in your "Utility" program group. The one in the middle.

If your run it the a window like this will open

4. Then on the terminal prompt type the following code (press "return" after each line):
$ cd /usr/bin
$ sudo ln -fs clang llvm-gcc-4.2
$ sudo ln -fs clang++ llvm-g++-4.2
5. After that, you should close everything then restart your MBA.
6. Then you can try again the process to install.package in R from tar.gz file. Use the menu to do this if you're not familiar with the command line.
Hope it helps.