I am sure that some of previous questions about this have been answered. I just thought this post (or re-post) might be useful for new user that come up with this problem .
I use OS X Lion (10.7.2) and previously had problem in installing Hydromad Package with conventional way (provided in the web site). First I thought It was completely installed by downloading the ./hydromad_0.9-19.tar.gzfrom http://hydromad.catchment.org/src/contrib/, because the "hydromad" was already listed in the package list. But, it turned out to be not completely installed, since I kept getting errors after loading it.
So I asked this problem to Statistics and R Communities in Google Plus, and surprisingly +Jesse Hamner(from Univ of North Texas) answered.
Based on his answer, I did the following steps:
1) basically Mac User have to download Xcode from the App Store (only for Maverick user).
For those with older OS X (which was also my case), you can go to https://developer.apple.com/xcode/ (but you have to spare sometime to register as App Developer first and off course you must have an Apple ID). After that, you can easily choose the right Xcode version based on your OS, download it, install, and reboot.
(the opening page, choose "download previous version of Xcode)
(after register as app developer and sign in Apple ID then you can choose the correct Xcode version)
2) Then you can start RStudio and install all the dependencies with: install.packages(c("zoo", "latticeExtra", "polynom", "car", "Hmisc", "reshape"))