Hi Juan,
I don't have a Mudtank RM file, but the easiest way to set one up is to click on the + button in the Reference Materials Browser of iolite, and select "New Empty Ref Mat". You can then fill in the metadata basics. For the values, you can just assemble your values in Excel in a spreadsheet with four columns:
Name, Units, Value and Uncertainty
You can then copy and paste the values (excluding any headers) into the RM's Data Table.
Then save your RM file by clicking the Save button in the top right of the window, and you should then have the RM available for all experiments.
If you do put together a Mudtank RM file, would you mind posting it here? I'm sure there are others out there that would be interested in it too.
Kind regards,
Bence