Assert::Throws will only work with a method that returns void. You're attempting to use it with a method that returns a float.
The simple solution is to wrap the property read in a method, and assert that the wrapping method throws the exception.
void ReadMyProperty()
{
float ignored = this.MyProperty;
}
System::Type^ type = System::NotImplementedException::typeid;
NUnit::Framework::TestDelegate^ delegateToTest =
gcnew NUnit::Framework::TestDelegate(this, &(MyClass::ReadMyProperty));
Assert::Throws(type, delegateToTest);