The sim files are designed to only be opened from within Universe Sandbox ².
If you're not seeing it within Home > Open > My Sims, then something is wrong. Do you have any sim files in there which were saved on a Windows machine? There's a known issue that makes sims disappear when Windows sims are put into the Mac folder. To test this you could temporarily move all of the files out of the folder and try saving a new one, then seeing if it shows up in Universe Sandbox ².
We have a fix for this coming in our next update, sorry for the issue.