I started a similar link above this one Rob.
It looks (to me) like Steam, in it's wisdom, only allows you to choose the "play offline" option when you are connected to the internet. Since probably 99% of the players that need to play offline, choose to do so because they have problems with their internet connection, it is a bit of a useless feature.
It's not that simple tbh, usually if you just don't have a net connection it offers you offline mode with no problem, just every now and then it decides that you first need to go online before you can set offline mode.
It's only ever happened to me when a Steam update has been released so it's somehow tied in with that, but even Steam don't seem able to get to the bottom of it.
Curiouser & curiouser Kriss.
I live in El Salvador & losing internet connection is not uncommon. These are the only times I am desperate to play the game & I have to connect whilst offline.
Would it be possible for someone at FM to push Steam to put out definitive instructions to users as to how to access the game whilst offline? It would be a terrific help.
You could possibly work it the other way around. I have steam in offline mode all the time. The only time I changed it is when there was a patch. I also turn off the internet on my system as I run in full screen mode, as such avoiding the chance of a system update whilst playing. To date, touch wood, I am yet to have any issues of playing