After you wrote the functions in inc ( eg:- demo.inc) you need to specify them in Use path/Use File

In Silk Test-> Options-> Run time

Specify the inc file path/File name in "Use File" Text box

(or)

Secify the file Path in "Use Path" and added the file name in "Use File" text box

(or)

In your script file add code like

[] use "demo.inc"

and add the inc file path in Silk Test-> Options-> Run time -> Use Path