There are two things we can do to help Murdock:
First things first, the militia need to have some weapons and armor. In order for them to get their equipment, you need to convince Owen to start up his forge again. This won’t be that easy, though.
The blacksmith is marked on your map. He’s just behind Murdock. Approach the locked door and try to open it to trigger an exchange with Owen. You should be able to talk your way inside.
The easiest way to handle this one is to just promise him that you’ll look for his daughter in the castle once you turn back the invasion. The problem with this is that Morrigan dislikes you giving more “false" hope and Sten assumes it is an empty promise. We actually will find her later, but that’s another quest. If you make a promise to raid the castle, he’ll fire up the forge. This is the only way to get him to do it.
You can also kill him right now and get into his stash with a key. You could also just pickpocket it or stumble across the stash, so it seems a little pointless to me.
You can also just leave and tell Murdock that you can’t change his mind.
If you get him to fire up the forge, the militia should have better armor and weapons in the battle.
Murdock really wants a veteran soldier on the field that night. Dwyn is such a soldier, but he refuses to join the fight. Go over to his house. It’s just past Murdock on the little dock area over the lake. Head on inside and talk to Dwyn and his two thugs.
He doesn’t want to fight, so you have to convince him. You can just hire him for 100 silvers. If your persuasion works he’ll go down to 50 silvers. Although if your persuasion is good, then you could probably just convince him without paying him.
You can also use intimidation to get him to join the fight, but he’ll attack you if it fails. I passed it with 2 points in Coerce though. If he agrees to join the fight, you can go back out and report it to Murdock.
You can also just kill him. If you kill him, you can just tell Murdock that he wouldn’t come around.