The namespace to import might not be identical to the filename of the .dll or .gha assembly. You can inspect the GrasshopperTeklaLink.gha after referencing like so (for the KangarooSolver.dll here):
import System
def inspectAssembly(assemblyName):
""" Inspect a .NET assembly and print its available types. The
assemblyName is the full name of the file to inspect including
its extension e.g. "Kangaroo0096.gha" """
# Load the assembly and print its full name
assembly = System.Reflection.Assembly.Load(assemblyName)
print assembly.FullName
# Iterate the types in the assembly
for i in assembly.GetTypes():
print i
inspectAssembly("KangarooSolver.dll")