Getting your Tax Card (Luxembourg)

Dear All,
I'm working for a company, but my contract is handled by Randstad, that made me sign some papers regarding the Tax Card, which I never got.

Now I'm pretty pissed to see my salary lower than expected because i'm being taxed at the maximum, instead of 17%.

Any suggestion where to ask? I live in Belair, I heard the tax offices are different per every single zone, but i'm still not sure.

Thanks,
Alessandr