Perforce in Unity 4.5 missing all the settings, says "Executable error"

We were using Perforce in Unity, but when we upgraded to Unity 4.5, two of our team members had their Perforce support broken. When we go into the Version Control settings, most of the settings are missing and it says "Version Control executable error".

Any ideas? This is severely hampering Unity's usefulness to those two team members.

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