Your wallet's xpub is currently not shown on the UI (although we are thinking about it).
This is mainly as it is quite hard to explain to users that if you give anyone your xpub they can see all the contents of your wallet FOREVER.
Anyhow, if you need it you should be able to work it out by taking an
offline copy of:
https://dcpos.github.io/bip39/and using your wallet words.
Check that the first address generated matches the first payment request in your wallet.
The derivation path for the wallet is shown in MultiBit HD in the Manage wallet | Wallet dashboard | Wallet capabilities section.
(It is different for MultiBit HD wallets and 'Trezor/ Keepey' wallets.