This achievement is story-related and can't be missed. Once you dock the Ark Hyperion at the Nexus, you'll need to speak with the necessary leaders. Once you do so, one of your squad-mates will tell you that the ship is ready. You simply need to go to the docking area to progress the story.
You will also gain access to a few quick side missions during this first visit to the Nexus, but you can proceed at your own pace prior to going to the docking area.Edit:
As seen in comments below, it looks like this achievement (and others) are at least partially bugged. I earned this during the early trial period without any issues, playing on Normal using a custom character.Jan 0
commented below with a possible work-around:
It popped for me when I reloaded an autosave at the beginnng of nexus and played to that part again in 5 min. you should have this auto-save as well. good luck!BarryWhite85
also commented with another possible work-around:
I found that this doesn't unlock along with a few other achievements if you don't allow your game to connect to EA servers before you click Resume / Load etc. Mine didn't connect on one gaming session and I missed about 2-3 achievements along with acquiring no further action on the hardball / terminator etc even though I had killed loads. So I will have to do them achievements when I do the Insanity Playthrough