You fill the cooling system from the expansion tank ... on the right hand side of the engine compartment as you look at it from the front.
Radweld isn't a great idea though ... it tends to clog the radiator and may make you more prone to overheating. Plus, your slight leak might be from anywhere - could be a leaky hose, or a leaky filler cap even - both easily fixed.
Best thing to do is to wipe clean all hoses and the filler cap area, and watch the coolant level in the expansion tank. How much are you losing over how many miles? Are there any obvious leak points?
As you lose coolant, you'll need to refill it up to the MAX level on the expansion tank. Be sure to use an appropriate mixture of anti-freeze and coolant, or use a "ready-mixed" coolant.
I've had a slight leak for about a year ... spotted on my last MOT. It's been fine - just have to top up the coolant every week, using Halfords ready-mixed.