A user defined function (UDF) can be used to implement the Casson fluid model. The Casson fluid model is given below:
A UDF incorporating the equations above is included in the file that you can download here. This file also contains the UDF for defining the time variation of the inlet velocity. You need to use this file for the Casson model and not the one provided in the tutorial. This is necessary because all UDF's need to be in one .c file. You could also use this .c file for all your cases whether you are using the Casson model or not.
In order to implement this file in Fluent, in the top menu bar, under "Define" → "User Defined" → "Functions", choose "Interpreted". Find the .c file in the directory, and choose "Interpret". Then under materials, double click on the fluid you defined, and for viscosity, choose "User Defined". Then choose "casson_viscosity". You assign the inlet velocity as in the tutorial.