I don't know how well you know Balerno but here goes.
When you turn off the Lanark Rd W at Balerno, take 1st left after the high school onto Bavelaw Rd. Stay on this road and it continous into Mansfield Rd. Follow this road all the way uphill, You can follow it on Googlemap.
When you get to the end of the houses you will see a bus terminus at the top on the right at Cockburn Cresc', you carry straight on ahead taking you up to the countryside. As you follow the road up past the vetinary surgry on the S bend you come to a fork in the road, keep to the left.
As you are still climbing (Watch for the potholes...winter damage) the road starts to level out. Follow on for a few hundred yds and you will see a car park on your left, It's signposted.
Once you have parked the car go back out to road and turn left, about 4 or 5 mins walk till you come to a bridge, Immediatly on far side of bridge is a gate on the right. Through the gate along wooden walkway and a few mins walk leads you straight to the bird hide.
It is alway unlocked, usual countryside rules, close gates behind you and doors and window latches in the bird hide when your finished
If you have time can walk through the other end of car park, it takes you down to Threipmuir reservior, you might get lucky and see some wildlife there, or scenry at least. Pick a nice day for it.
Have fun