using libsvm classifier in Weka and heap size
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problem in working with WEKA 3.6

I want to utilize libsvm.jar but the error "not in the classpath" occurs when I run weka with the command prompt as below:

java -Xmx900m -jar Weka.jar

please note that we use libsvm library when I run weka in normal mode (with the initial heap size and without using command prompt). 
besides, I use windows 7-32bit.

help me plz I am running out of time.
thanks