The plot S3 method plot use matplot
to plot random forest model. You should add legend manually. This should be a good start:
library(randomForest)
model = randomForest(Species ~., data=iris, ntree=100, proximity=T)
layout(matrix(c(1,2),nrow=1),
width=c(4,1))
par(mar=c(5,4,4,0)) #No margin on the right side
plot(model, log="y")
par(mar=c(5,0,4,2)) #No margin on the left side
plot(c(0,1),type="n", axes=F, xlab="", ylab="")
legend("top", colnames(model$err.rate),col=1:4,cex=0.8,fill=1:4)