I am assuming Kiera is using it on the auto setting, in which case a neutral density filter will not work because the camera will simply increase the auto-exposure to compensate.
As ianmunt says, you need to set it to manual control and experiment with different exposures. If you have the Canon 450d (digital version) then nothing is wasted while you experiment.
You don't say whether you are trying to take pictures of just the moon (in which case follow the advice above) or whether you want a landscape that includes the moon. If the latter, then you will be forced to choose a longer exposure to get detail in the landscape (and the moon will be washed out). So here's what you can do:
1) Buy a graduated Neutral Density filter that reduces exposure to the sky half of your picture whilst allowing full exposure of the bottom half.
2) Do as I do and take two pictures - one exposed for the moon and the other exposed for the landscape. Use digital editing software (eg GIMP) to combine the two.