How do we waterproof a cement slab floor that has moisture coming up?
It sounds like you have two problems.  

Water is coming in through the space where the wall meets the floor.

This can be mitigated by making sure the surrounding terrain is sloped away from the walls, ensuring that surface water flows away from the structure.  You might also be able to dig out the foundation and seal the soil/structure boundary with a heavy plastic such as Delta Board.

The other problem is likely what's called rising damp.   This is water underneath the foundation that wicks up through the concrete.  The solution is expensive.  You need to excavate the foundation, Install weeping tile which drains to a safe location.  This will help keep ground water out from underneath the foundation.

Since you say this is a barn, the foundation may not be that deep, and it's something you could do yourself.  Just be sure to fully research (or ask questions here) how to do it.