Holding down the slew keys doesn't help
I was trying by holding down the slew keys for a long time, but I still couldn't break the lock.
Also it looks like the lock key doesn't work for me when I try to lock the ground (KC) nearby the targets. Initially I thought that it's because the image moves to fast to allow a lock, but now I realized that even if I keep the plane very steady aiming somewhere nearby the targets, I still can't lock the ground (KC). I can only lock a target (AC) if I move over it, but this is most likely the autolock working, not my Tab key.
I didn't touched the default keyboard mapping (I only changed the mapping for my joystick) and the Tab works without problems when locking in a2a or antiradar.
Once I reach home, I'll completely remove both lomac and lockon, then install them fresh using only defaults, try again and let you know.
Thanks everybody!