BTW is correct. Just look hard...they tend to be in basements and lofts, and will be "gaming parlours" rather than Internet Cafe's...but all have great gear, super-fast graphics and broadband connections. As a rule they are poorly signposted.
If you want a specific address, then head N on Nathan Road, take a right into Humphries Avenue, stay on the Southern Pavement, and after about 30 metres, look for a sign directing you up. It's one of dozens in TST, and the "membership" is nominal (often free) and rates about HKD20/hr.