The R2admb package provides an interface between R and AD Model Builder. There is a stable version on CRAN, and an old (2012 November) version on R-forge, but active development is now happening here on github. In order to install the most recent development version, install the devtools package from CRAN and then:

library("devtools")
install_github("bbolker/R2admb",subdir="R2admb")