That reqiurement is actually from vanilla OpenTTD, not even our citybuilder mod.
For town to grow it requires at least one active station in town vicinity (if you want you can see exact zone with our modded client).
As ST2 said players usually do it by starting some pax service in town (you need at least 5 stations for max growth speed anyway).
Also all that only applies to classic CB servers, new CB ones (#3 and #4 currently) work differently. You'll still need to tranport passengers and mail in town, for faster growth. But there are no tricky requirements on station amount/placement so if you're new to citybuilder I'd recommend you to play there.