General tip: Save and quit each time instead of pressing the home button when you're done playing, so the game will actually check if it's connected properly when you return.
Not quite sure which level this unlocks at, but I bought it at level 9. Costs 19 LP. Don't buy the 29 LP upgraded version; it won't work.
Find an empty lot then scroll far right until you see Pro Skater's Dream Pad. After the building is finished, make sure you are connected to the servers properly by checking the leaderboards in the menu, then either create a new Sim or move one into the dream pad, and the achievement should pop.
Other requirements that we previously thought mattered, but don't actually seem to are: 1) creating a new Sim instead of moving one in; 2) Sim should be sporty; 3) standard lot vs premium lot; 4) gender; 5) using LP to finish the building
I personally unlocked this achievement by building on a standard lot, using LP to quick finish, and creating a new sporty girl Sim within, and I got the achievement on my first try. However, general consensus from the comments below, especially the comment by xxB3averHunterx indicates that none of these other factors matter besides being online when any type of Sim somehow ends up in the dream pad:
"I finally got it to pop this morning. Bought my 3rd skater house and double checked the leaderboard just before creating a non-sporty Sim and moving him in. It popped right after I moved him in."