[Dridzt]

I believe you could accomplish that with the Load conditions without writing a custom trigger.

On the Load tab you'll find "Instance Type".
Checking it allows you to pick one from dropdown, clicking it again presents you with a multiple checkbox section for all instance types.
Just check the ones you want the aura to load in and leave the rest unchecked.

Custom trigger is also relatively easy.
You'd want
Custom Trigger:

function()
  return not IsInRaid() and true
end

Custom Untrigger:

function()
  return true
end

Generally speaking the dedicated WeakAuras forum is a better place for usage tips and self-help from the community.

This set of pages is also a good resource:
http://www.wowace.com/addons/weakauras/pages/