An iso is burned in a special way. The iso file is literally a CD image, not a regular file.
Common CD Burning Options
Iso images should be burned in Disk at Once mode (DAO).
MEPIS CDs expect that you are using 700MB CD media, not 650MB media.
Easy CD Creator
If you are using Roxio Easy CD Creator, you need to select to Record CD from CD Image and then select the iso file to burn.
It may be necessary to select ISO Image Files (.iso) in the file type combo box.