Are you still having the same problem? The exact same thing happened to me, except I have a 325xi. I don't know if the 528's work differently or not.
Anyway, I too was testing out my DSC and nothing happened when I held the button down for 3-5 seconds. However, after holiding it down for 10-15 seconds, the light went out and I couldn't get it back on by pressing the button or holding it down, just like you. It seemed to me that DSC was completely off but I couldn't tell for sure. For all I know, it could've been on. Anyway, I stopped and restarted the engine immediately to see if that would fix it and it did not. However, tonight I just went out and the light was working fine again when I pushed the button. I'm wondering if it just takes time to reset.
Maaz