Report Services: Creation of functions from Report Services.

Buenos días,

Quiero crear un campo dentro de un modelo que sea una expresión, esta
expresión quiero que sea un IF..
IF campo is null then campo2 else campo

Me gustaria saber como decir IS NULL (isnull).
He visto que existe esa función en SQL Server pero alli ni me deja utilizar
nada.
He probado:

IF(campo isnull; campo2;campo)
IF(campo is null; campo2;campo)
IF(campo = null; campo2;campo)
IF(isnull(campo); campo2;campo)
y nada de nada...
¿Alguien ha utilizado esta opción de Report Services?

********

Good morning,
I want to create a field within a model that is an expression, this
expression I want that it is a IF.
IF field is null then field2 else field

I want to know like saying IS NULL (isnull).
I have seen that that function in SQL Server exists but nor it lets to me
there use nothing. I have proven:
IF (field isnull; field2; field)
IF (field is null; field2; field)
IF (field = null; field2; field)
IF (isnull (field); field2; field)
and nothing don't mention it…
Somebody has used this option of Report Services?

Thank / Gracias