Update: I think the problem was spacing and quotes. This worked (on windows):
Rscript -e "a <- 3+3; cat(a,'\n')"6
On Mac, you have to escape the escape character:
Rscript -e 'a <- 3+3; cat(a,"\\n")'
You can also put each expression separately.
Rscript -e "a <- 3+3" -e "cat(a)"