Take a look at this memo from Simon Urbanek on the R-sig-Mac list in February:
https://stat.ethz.ch/pipermail/r-sig-mac/2013-February/009967.html
"Ah, hold on - you are on Mountain Lion and you may have installed R with 32-bit default since 10.8 was omitted from the list of supported 64-bit systems at the time - try installing latest R 2.15.3 RC from http://r.research.att.com/"
You should also make sure that the RMySQL version is appropriate for version 2.15 since CRAN would now be serving up a version that expects R 3.0.0 to be running. Actually that should not be the problem since RMySQL_0.9-3 is in the 2.15 binaries trunk at the UCB CRAN mirror: http://cran.cnr.berkeley.edu/bin/macosx/leopard/contrib/2.15/RMySQL_0.9-3.tgz