[wow001]

I am running my home diy project and have been entering the materials that will be used i.e. paint, telephone sockets etc.
When I assign these items as materials, I cant place a quantity i.e. 5 slave telephone sockets or 25 litres of paint, (the field is active but will not allow any input) so that I can assign a number or quantity of the material to a task.

[Lizard]

That's intentional. The idea is that in the resource view, you just say "Gallons of Green Paint" without specifying a quantity. Then as you're going through assigning material resources to tasks, you'll figure out that your living room will require 4 gallons of paint, so in the Task: Assignments inspector, edit the "Assigned Amount" to be 4. Perhaps the bathroom only requires 1 gallon, so you assign "Gallons of Green Paint" to that task too, but leave the "Assigned Amount" at 1. Then when you switch back to resource view, you'll see that the Green Paint's units have been updated to '5'.
This way, when you change what you need for one task, you don't have to remember to update the total as well -- OmniPlan handles it for you.