The command for use YUI is :
java -jar yuicompressor.jar original.css -o original.min.css
Try this but specify the path of yuicompressor.jar :
@echo off
echo Combining.....
type lib\css\header.css>>stylesheet.css
type lib\css\footer.css>>stylesheet.css
java -jar /path/to/yuicompressor.jar stylesheet.css -o stylesheet.min.css
echo Finished!
PAUSE