An ISO is an image file, so you use the 'Write Image File To Disc' option.
Once you've loaded the ISO in 'Write' mode, it'll say if it's bootable or not next to the 'File Sys' ('File System') label. Either you'll see the word 'Bootable' or you won't.
If it isn't bootable, you'd need to recreate (rebuild) the ISO and make it bootable... or just try and find one that is bootable already.