Using data validation values as dynamic feeds in a Getpivottable command.

You can achieve this by modifying the GETPIVOTDATA function to dynamically reference the value in cell B3. The key is to replace the hardcoded [Administration] value with a reference to B3. Here's how you can do it:

Adjust Formula and use: 

=GETPIVOTDATA("[Measures].[Sum of Posting_Amount]",'month Facility Departmental'!$A$6,"[Wolfs_Expenditures].[Fund]","[Wolfs_Expenditures].[Fund].&[001]","[Wolfs_Expenditures].[Object]","[Wolfs_Expenditures].[Object].&[0201]","[Wolfs_Expenditures].[Object Series]","[Wolfs_Expenditures].[Object Series].&[200]","[Wolfs_Expenditures].[FYFP 1]","[Wolfs_Expenditures].[FYFP 1].&[20257]","[departementaldescr].[Dept Name]", "[departementaldescr].[Dept Name].&[" & B3 & "]")