First make sure it's in /opt/zimbra/conf (though I'm sure the directions you got with the trial told you this).
cp ZCSLicense.xml /opt/zimbra/conf
but what they might not say (I haven't requested a trial in a long time) is about the ownership/permissions:
chown zimbra:zimbra /opt/zimbra/conf/ZCSLicense.xml
chmod 444 /opt/zimbra/conf/ZCSLicense.xml |