Both sides are open. Coming from braehead you'd turn left off renfrew road and go on a long road with a bend in it past a horrendous smelling (at times) garbage/recycling processing plant. Then there's the roundabout way at the end of that road. After going straight through there you need to turn right down a small dead-end street, then there is a ramp down (just after the street has ended and you're up on the kerb). Head down there and press the buzzer and they'll open it for you.
Coming the other way had me lost many a time, are you commuting back through the other way too? If you're unfamiliar with it it's a total pain to work out how to get in to the southbound tunnel