Open a command prompt (click Start, select Run, type "cmd" and press enter)
Type "java -version" and press enter

The first line of output should be something like

java version "1.4.2_06"
or
java version "1.5.0_06"

The two numbers after the underscore aren't that important.

If the version number is Ok, then change directory to where you have MorphExchange.jar and type

java -jar MorphExchange.jar