To import a watch-only wallet, you can simply add a new 'account' and then enter either an address or the master public key (xpub).
But AFAIK, you can not create unsigned transaction with mycelium.
The 'offline wallet tool' from the drop down menu is for importing private keys / xpriv / seeds from a printed version (e.g. from a paper wallet).
This is not for creating unsigned transaction on an offline mobile.
I was writing some articles recently on cold wallets, this is the exact thing that stopped me from featuring Mycelium on my list.
However, you can create an offline wallet with Mycelium, just put it in a phone with no cellular or wifi or any kind of data. Just won't be able to create those offline transactions.