1) As I originally asked, where are you putting this code?
It's possible, depending on where it is, that it's loading before the functions it uses are created by the gamemode or addon.
2) Try a print command inside the function too, to see if it's working right when it runs, if the hook is even running.
function AddHpToLevel(ply)
print ( ply:Nick() .. " level is ".. ply:getDarkRPVar('level') )
if ( ply:getDarkRPVar('level') >= 60 ) then