Sounds like OmniMech is attempting to use the LW version 5 beta which requires Java 1.6. Unfortunately when LW 5 'full' version is released, Tiger OSX 10.4 users will not be able to use it.
If you are using Leopard OSX 10.5, then go to your Java preferences (found in utilities folder), and make Java 1.6 the first or second to use in priority.
Setting up Java for Leopard