It depends on whether you wish to return values from the JAR file or not. If you do not, and just wish to execute a method then you can use a "Shell" (or ShellExecute) from VB as follows:
Shell("java.exe -jar " & <yourJARFile>)
If you do need to return a value then you will more than likely need a bridge - similar discussion here (for VB.NET, but principle is the same): Can you use Java libraries in a VB.net program?
Based on that most bridges that people have created are for .NET and not for VBA/6 so you would maybe need to create your own somehow, or perhaps create a .NET DLL that is then consumed by your VBA code.
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