There's so much cheese for radiant damage out there - boons that grant you bonus damage when you damage radiant vunerable targets, ways to impose radiant vunerability, etc etc. Now Warlocks aren't the absolute best at radiant, but there's definitely some things you can do.

I'll assume you don't have a DDI subscription, which is too bad, because most of the best stuff is in there. Here's a few simple methods.

1) Star Warlocks are great warlocks. Not in terms of damage per round, but in terms of performing the way you might want them to. See if your GM will let you use material from here. IA! IA!

2) As a general rule for lock advice, you'll want to focus on getting..
....Things cursed as often as possible
....Getting your pact boon as often as possible
Therefore, its generally a good idea to pick up the Double Implement Spellcaster feat, and for weapons, look into the Bloodcurse Rod and/or Rod of Corruption.

3) If you DO have access to DDI or Dragon Annual, you can become a Servant of Caiphon, and take Arcane Implement Proficiency (Heavy Sword) and grab a Radiant Weapon. Now all of your attacks can deal radiant damage, and you get 18-20 Crit range. Melee warlock!