I totally agree with Dennis about using Instance Access. However, if you still want to put the constraint, you have to select the reference set selector put the constraint (see below) on the reference set selector instead of the selection form:
[Name != 'MxAdministrator'][Name != 'Developer']
Note: you should use 'And' instead of 'or' in your constraint.
It is safer to set your Instance Access, in the Security section. You can find your answer here