Custom Rollup Formulas On Analysis Services 2005

Hello all, I´m a beginner on AS2005 (but I know pretty well AS2000), I migrated a cube from AS2000 to AS2005 but in this cube I have a Custom Rollup Formula on two levels of my Time dimension (month and week), an example of the week level formula is the next:
iif( [Time].CurrentMember.Name = [Time].CurrentMember.NextMember.Name,
null,
iif( [Time].CurrentMember.Name = [Time].CurrentMember.PrevMember.Name,
Sum({[Time].CurrentMember.Children,[Time].CurrentMember.PrevMember.Children}),
Sum([Time].CurrentMember.Children)
)
)
<<this formula overrides the week aggregations>>

and I really need this but I cannot find on AS2005 where to do the same thing, someone can tell me where can I define Custom Rollup Formulas and how?.
Thanks in advance!