This would appear to be one of those urban myths which have sprung up because so many people are looking for ways to pass urine tests in the workplace.
Here is some information I found.
Charcoal pills are mostly made from coal, wood, peat or petroleum and are taken for a lot of medicinal purposes. Some people take them for exess gas. You take the pill and your exess gas binds to the surface of the charcoal - you swallow the pill and the gas is gone.
You can see why people think it would work for drugs. And a doctor can administer charcoal pills if he thinks you have swallowed poison.
This is because charcoal pills work in your digestive system.
However Meth does not stay your digestive system but goes to your cells. And can stay there for up to 3 days. Have a look at this information video on how long Meth can stay in your system
You can try all sorts of things, but using charcoal pills to help you pass a urine test has never been scientifically proven.