In order to attempt a dispel, the dispelling player first nominates one of their Wizards to make the dispel attempt. Unlike spells, dispels do not have a range and never require the Wizard to see either the target or the caster – you can simply nominate any Wizard on the battlefield.
Wizards cannot try to dispel spells if they are fleeing or not on the battlefield. If the dispelling player does not have an eligible Wizard to attempt a dispel their army can attempt to dispel the spell instead.