I opened the model and edited the modeldef.xml code for some gauges, but it looks when I saved the model the new code was not applied.
When I choose "Save..." in the ModelDef.xml dialog, it offers to save it as a separate file. So how do I apply an edited modeldef.xml to a model?
No, when you load the model MCX will load the modeldef.xml from the path in the options. If you make modifications those are kept in memory. When you export the in memory version is written to a temp file on disk and used for the compile. Afterwards it is removed again. So the file specified in the options is not changed or so.