wasmdis module-file output-directory [regex] [tier]
where module-file is the path of a .wasm module,
output-directory is a directory for storing the output of the tool,
regex is an optional .NET regular expression used to filter functions,
and tier specifies which spidermonkey codegen tier you want. ion is the high quality background tier which should generate smaller/faster code, but the other tiers matter too.
You need to have jsvu installed and make sure you've pulled down a build of spidermonkey using it so that you have a ~/.jsvu/sm.