adjusting the rod on the backside of the pedal that goes to the master cylinder changes the pre-load on the cylinder. you most readily notice the difference by the 'length' of the pedal (short/long). Adjusting it too much one way can make the clutch slip, as there is always pressure on the system keeping the clutch from ever fully engaging the driveling again (the clutch arm needs to be shortened). This happened to me in my turbo swap the first time I took it out. 5 minutes upside down under the dash with some open-ended wrenches fixed it.
That is to say that's what your problem is. If you do this and it's still slipping, you need to look for other issues. Old/bad clutch disc, etc