Try this then redo your code:
cross.m$Observations <- factor( cross.m$Observations,
levels=c("Underweight","Normal","Overweight") )
cross.m$Predicted<- factor( cross.m$Predicted,
levels=c("Underweight","Normal","Overweight") )
conf <- table(Predicted, Observations)
library(caret)
f.conf <- confusionMatrix(conf)
print(f.conf)
Ordinary matrix methods would probably not work since a caret confusion matrix object is a list.