Everything seems to be correct.
I've never tried that in amxmodx 1.5 but let's just assure:
- you're playing counter-strike 1.6 (and not condition zero)
- "STEAM_0:1
xxxxx" - this xxxxxx is a valid steam id
- check if you get any [model error] messages in the console
I'll try to test it today in a condition zero server.