How do I enter Polish characters?

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http://askubuntu.com – I just installed Ubuntu 12.04 and... I've got a problem :) I'm living in Poland, so I want to write texts in polish with polish chars such as ąćęłńóśźż, too. Unfortunately, it's impossible. We write polish chars by pressing AltGr and chosen key (e.g.: AltGr + A = ą, AltGr + X = ź, AltGr + C = ć, etc.), but when I press AltGr, it isn't like pressing AltGr, it's like pressing Alt - I can open menus with their mnemonics, and I can't enter polish chars. Btw. - I've got, of course keyboard layout set to Polish. So my question is: how to fix it? (HowTos)