Can someone tell me how to add a masterwork on a weapon please?? And how much do they influence the stats??

you can't add it
there's a chance that after a weapon is being crafted, it'll be masterwork
it does influence a lil bit, not as much as SA, but still quite useful

oh i see..thx

When U are crafting there is chance to get foundation weapon/armor - 10%; and small chance to craft 2 items - 2% (the items won't be foundation :/ )

After U have foundation item U can exchange it for masterwork item ->
U have to go to the Merchant of Mammon and buy Mammon's Varnish Enhancer

for B grade Foundation items you need 1 Varnish Enchanter,
for A and S you need 2,3 up to 7 for S grade weapons. The Varnish Enchanter costs 100K Adena not AA

then go to the Blacksmith of Mammon with foundation item and the Varnish Enhancer and he will unseal the bonus =)