Hej!
I had the same issue and when running the following command in R - as suggested there: http://socserv.socsci.mcmaster.ca/jfox/Misc/Rcmdr/installation-notes.html - I realised that there was a remnant tcl/tk 8.5 installation on my system.
system("ls -ld /usr/local /usr/local/lib /usr/local/lib/libtcl*")
drwxr-xr-x 12 root wheel 408 24 Oct 02:21 /usr/local
drwxrwxr-x 61 root wheel 2074 30 Oct 22:43 /usr/local/lib
-r-xr-xr-x 1 root wheel 4820229 21 Oct 2008 /usr/local/lib/libtcl8.5.dylib
-r-xr-xr-x 1 root wheel 1419604 30 Mar 2013 /usr/local/lib/libtcl8.6.dylib
-rw-r--r-- 1 root wheel 11072 21 Oct 2008 /usr/local/lib/libtclstub8.5.a
-rwxr-xr-x 1 root wheel 4824 30 Mar 2013 /usr/local/lib/libtclstub8.6.a
Removing the old 8.5 files fixed the problem for me.
HTH