I assume you are referring to a Municipal Utility District tax.  These local benefit districts typically impose taxes to pay for local services and infrastructure.  If the tax is based on the value of your property, then it's deductible as a real estate tax; just add the MUD tax to your other real estate taxes and enter the total under real estate taxes.  However, if the assessment is not based on value, it's not considered a property tax and is not deductible.