You can't. It doesn't play or read .iso files (at the time of writing, 0.9.4) and I don't see the need for this anytime soon, but you can easily convert SACDs to stereo or multi-channel DSD files, either in Sony DSF (.dsf) or Philips DSDIFF (.dff) format. Both formats are playable in Resonic.
All you need is a tool called sacd_extract, which also comes packaged with a GUI here: https://www.sonore.us/iso2dsd.html
If you prefer the command-line you can do something like:
sacd_extract --2ch-tracks --output-dsf -i "your input sacd.iso"
for stereo .dsf output
sacd_extract --mch-tracks --output-dsf -i "your input sacd.iso"
for multi-channel .dsf output
Use --output-dsdiff to create .dff files.
Or use the GUI
After a while you'll end up with a new folder that contains a bunch of files. Quite neat.