An MLS can only be added if Side is already a member of the MLS. As an agent, you can be a member of local MLSes that Side is not a member of.
If you do not see a specific MLS vendor option, you can reach out to chat support or email help@side.com to see if that additional option is available to add to your Side App.
Side does not guarantee that every single MLS that is requested will be added, only existing ones that Side is a part of. However, Side is always looking into adding more when there is enough of a demand.