Currently the recommended way to install the Hmsc package is to use install_github function from devtools package, available from CRAN. Additionally, since Hmsc is dependent on the BayesLogitpackage to support observation models for count data, and BayesLogit was temporary removed from CRAN, it is necessary to install it before installing Hmsc.
The following lines should sucessfully install Hmsc to your R in most cases:
library(devtools)
install_url('https://cran.r-project.org/src/contrib/Archive/BayesLogit/BayesLogit_0.6.tar.gz')
install_github("gtikhonov/HMSC")