You will need to combine the two calculations : (parellelperiod and current member to make the set within the AVG function).

WITH MEMBER [Measures].[Last 6 Members Average] AS

AVG(

(

{

PARALLELPERIOD([Order Date].[Date].[Date], 5, [Order Date].[Date].currentmember) :

[Order Date].[Date].currentmember

},

[Measures].[Internet Order Quantity]

)

)

SELECT

{ [Measures].[Internet Order Quantity], [Measures].[Last 6 Members Average] } ON COLUMNS,

[Order Date].[Date].[Date].[ALL].children ON ROWS

FROM [Sales Cube]

Fitz