Adding customized fields, function and constants to SSRS

Hello,

I am using SSRS for Dynamics AX 2012.

I need to add customization to SSRS report.

I think that this is because SSRS is built especially for AX 2012 (i.e. the SalesInvoiceReport).

I need to add, i.e variable on report.

I open the report designer + right click outside the design area + properties + variable. put variable "myABC", value = "ABC"

add an expression on textbox field:

=Variables!myABC.Value

and get the error:

Value is invalid, and the name of the value.

Simmiliar message as I create a code (I am doing code.myFunction() in expression).

MyFunction may look like this (on code section):

Public Function MyFunction() As String

  Return "ABCD"
End Function

BTW, 

1. How can I call function from SSRS, when report is initialized?

2. The above code should be in VB. Can I do that on C# (VS 2010)?

Thanks :)