why can't i get online with entrenchment?

ok ive just bought the new entrenchment, and this may sound very noob of me even though i've been playing SOASE for a good 8 months now.

i was wondering, when i go online and i click on a game to join, something comes up that says that i can't join because i have different mods to the other player...

is this because i bought entrenchment and the hosts may not have it or is it something else?

i appreciate the help

ps if anyone wants to play me i cant host but if you do im on at about 8 most nights and called "Geminon"