You should find a repo that contains the package you want to install. A good repo for Galileo can be found here: How to enable the Yocto-provided package management capability. You can check if the mono package is in that repo, if not you should fine another one (another option would be to post a request in that thread to add the package to the repo).
I found this other thread where there is another suggestion about mono, you might want to take a look at it as well: https://communities.intel.com/message/277008#277008
This is for edison, I am looking for Galileo