Take a look at the write.csv or the write.table functions. You just have to supply the file name the user selects to the file parameter, and the dataframe to the x parameter:
write.csv(x=df, file=”myFileName”)
You need not to use even the package “tcltk”. You can simply do as shown below:
write.csv(x, file = “c:\myname\yourfile.csv”, row.names = FALSE)
Give your path inspite of “c:mynameyourfile.csv”.