The Spline2 function is part of the Data Analysis Extension Pack (DAEP). You have to install that extension pack (once on your machine). If it is already installed on your pc, the function should be readily available in Mathcad, nothing else to do but use it.
To check if it exists, you could type
Spline2(1)=
If you get an error message about the number of parameters, then the function is defined, and you can use it.
Alternative is that you get an error message stating that the function is undefined, in that case it is...
Note that the function name (as defined in the DAEP) starts with a capital S.... The function spline2 is undefined.
Success!
Luc