I am using latest beta but it is ignoring a custom XSLT in MultiMarkdown settings and looking for memoir.xslt irrespectively.
The custom xslt file is in ~/Library/Application Support/MultiMarkdown/XSLT
If I remove memoir.xslt from that folder I get an error saying it can’t find memoir.xslt so it is obviously looking there, but not for the file I specified.
Setting name is case sensitive, it should be “XSLT File”
None of the other settings end with a newline - MMD is sensitive to a blank line in the settings, it reads that as the end of the configuration; if you have a blank line in one of the settings it may be ending up above the XSLT File setting, causing that to be ignored.