See example for the element order in the vectors :
inpboxe2.png
To get the menu, just use the following code:
vlabel<-c('matrix 1','matrix 2')inp<-c('M','N')ans<-inpboxe2(vlabel,inp)
References
Lawrence M.F.,Verzani J., (2012) Programming Graphical User Interfaces in R,CRC Press Taylor & Francis Group