I know there's an ice skating ink on Cabildo at about Santos Dumont - not sure if it's an ice rink year long or if the change to roller rink at some point. If you're just looking for somewhere nice to go roller skating, you can rent blades around Plaza Holanda (the Rosedal) and they have a nice paved path that you can use. It's full of runners,walkers, joggers, ciclists, rollerbladers, skateboarders etc every afternoon. I think it's 8 pesos to rent for an hour.
Google "patinaje sobre ruedas" or just "patinaje" on google.com.ar and you should come up with some better links