http://wiki.eqemulator.org/p?Server_Rules&frm=Main
By default they are turned off, so set AllowMercs to true. If you're no SQL guru you can use HEidiSQL (it's free) to edit the values visually, but if you're keen on learning SQL (which is really quite a big part of the EQ Emu server system):-
Code:
UPDATE `peq`.`rule_values` SET `rule_value`='true' WHERE `ruleset_id`=10 AND `rule_name`='Mercs:AllowMercs';
ruleset_is=10 is the default EQEmu one, and is applied at the zone level (so different zones could use different rulesets, presumably so you can have pvp zones or not)...