Im trying to run a webserver in iMac but it doesn't work in port 800
You have to run as root to bind to the privileged ports (&lt;1024). You can run Mongoose via sudo, but then you will have a web server running as root. Not a good idea!

Changing user with run_as_user parameter also seems to be unsupported on Mac:

Mongoose[22361:507] The application with bundle ID (null) is running setugid(),
    which is not allowed.


Why don't you pick a port above 1024 e.g. 8081? Then you can run as unprivileged user.

For troubleshooting you can run Mongoose in Terminal.app like this:

sudo /Applications/Mongoose.app/Contents/MacOS/Mongoose