Assuming you have added a second tab and place the function there, I expect it is an argument type mismatch. Are context and head declared correctly?

How do I access a function in a separate file, and how does this separate file have to look like?

In the Processing IDE the Sketch | Add file option allows you to copy a file from another location into your sketch folder.

If all you want to do is split your sketch code over several files then click on the right-facing arrow icon to the right of the file tabs, then select New Tab. You will be prompted for a tab name so type one in without a file extension. This will create a new file with the file extension .pde

A sketch can have many pde files but as far as Processing it is the same as if all the code was in the main sketch tab. So there is no special format for code in another tab the only restriction is that the file must have a .pde extension.

The problem you are experiencing is either the imported file does not have the pde extension or as previously suggested the function arguments don't have matching data type.