You'll have to add the assembly name like this:
Type.GetType("MyProject.Domain.Model." + myClassName + ", AssemblyName");
To avoid ambiguity or if the assembly is located in the GAC, you should provide a fully qualified assembly name like such:
Type.GetType("System.String, mscorlib, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089");

You'll have to add the assembly name like this: Type.GetType("MyProject.Domain.Model." + myClassName + ", AssemblyName");
To avoid ambiguity or if the assembly is located in the GAC, you should provide a fully qualified assembly name like such:
Type.GetType("System.String, mscorlib, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089");