The extrafont package makes it easy to use system TrueType fonts and with PDF or PostScript output files, and with bitmap output files in Windows.

First, import the fonts installed on the system.

install.packages("extrafont")
library("extrafont")
font_import()
loadfonts()
loadfonts(device = "postscript")
# For Windows
# loadfonts(device = "win")

After installation, you load the extrafont package before starting RcmdrPlugin.KMggplot2.

library("extrafont")
library("RcmdrPlugin.KMggplot2")

If you want to see more details: Using extrafont - CRAN