Here is one way to do it. It uses the ability of R to construct file paths based on the platform and hence will work on both Mac OS and Windows. Moreover, you don't need to convert your xls file to csv, as there are many R packages that will help you read xls directly (e.g. gdata package).
# get user's home directory
home = setwd(Sys.getenv("HOME"));
# construct path to file
fpath = file.path(home, "Desktop", "RTrial.xls");
# load gdata library to read xls files
library(gdata);
# read xls file
Rtrial = read.xls(fpath);
Let me know if this works.