Added 2.5 oz Centennials into hop bag in keg. Been on C02 for two weeks at ~36 degrees and 10 psi. Not getting much aroma or taste yet. Any ideas what I did wrong?
Did you pull any pints right off the bat? Also have you purged the keg at all? I usually keg hop and I found (could be in my head) that if you start pulling pints too early and don’t let the hops sit for a few days, you will get a few REALLY powerfull pints, then diminished flavor and aroma after wards.
As for what you may have done wrong? I can’t think of anything you could do wrong. These are hops in a muslin or nylon mesh bag placed into a keg of beer, I can’t envision any way of messing that up! 
Maybe you aer just looking for more flavor than 2oz will give you, try 4 next time.
Are you letting the beer warm up from 36 degrees before trying it? I find I get better hop expression as the beer warms (maybe in the 45 to 50 degree range).
If that doesn’t help, as 802Chris noted, you might just need more hops next time.
My pale ale gets 3 oz of hops in the keg. I normally get a pungent hop character with hop debris for the first week or so. At two weeks the aroma and overall hop character is right where I like it.
I am not sure why you would not get any results from 2.5 oz…
I remember reading that the general solubility of the hop oil aroma compounds (linalool, geraniol, and many others) that we try and pull out of the hops during dry hopping is affected by temperature and the amount of alcohol in the beer. After fermentation, I dry hop in the keg using a very fine mesh stainless strainer at room temperature (68-72F) for about 4-5 days. Only after this step do I cold crash, fine with gelatin (if needed), and then carbonate. I’m pretty happy with the results.
I assume the hops smelled good before they went in? Lots of aroma?
My guess is that it was just too cold at the outset…I’ve found (as others mention) that dry-hopping cold doesn’t do a lot, it takes awhile for the character to show, and the character is muted relative to a beer that is dry-hopped “warm”. But per Denny’s comment, did the hops smell aromatic when they went in?
FWIW, I’ve added hops to a 38-40F keg and gotten good results (though granted it takes longer to extract). I also wonder if these were vacuum sealed, frozen hops, ie., hops in good condition, or hops just measured by the LHBS and put into a Ziploc bag. I ask because I used to buy hops by the oz from the LHBS when I started. Once I bought a vacuum sealer and bought hops in bulk from good hop retailers, the difference was astounding.
I stopped cold crashing so i add my keg hops warm and then put the keg in the kegerator to cool. They stay until it kicks. Results are good.
Same here.
Cold temps to start, so it will take longer as others have mention, that and the age condition of those hops. My process is to dh in bags in the primary so i don’t have to cold crash, then keg hop commando and leave at room temp for 4 - 5 days then into the cooler.
I throw a couple of big heavy stainless steel fittings into the bag so it sinks. (I boil them with the bag before adding the hops.)
Get the hops into full contact with the beer and that really helps, at least in my experience.
HTH-