Try:

SELECT          DataSource1.publisher,
                IIf(IsNull([CountProjectMGMT]),"na", Val(DataSource1.SumProjectMGMT/DataSource2.CountProjectMGMT)) AS AvgProjectMGMT,
                IIf(IsNull([CountDesign]),"na", Val(DataSource1.SumDesign / DataSource3.CountDesign)) AS AvgDesign 
FROM            ((SELECT   publisher,
                           SUM(IIf(IsNull([ps_projectmgmt]) OR [ps_projectmgmt]="na",0, Val([ps_projectmgmt]))) AS SumProjectMGMT,
                           SUM(IIf(IsNull([ps_projectmgmt]) OR [ps_projectmgmt]="na",0, Val([ps_projectmgmt]))) AS SumDesign
                  FROM     evaluations
                  GROUP BY publisher) AS DataSource1
LEFT OUTER JOIN  (SELECT   publisher,
                           COUNT(ps_design) AS CountDesign
                  FROM     evaluations AS Table_1_2
                  WHERE    (ps_design <> "na")
                  GROUP BY publisher) AS DataSource3
ON               DataSource3.publisher = DataSource1.publisher)
LEFT OUTER JOIN (SELECT   publisher,
                          COUNT(ps_projectmgmt) AS CountProjectMGMT
                 FROM     evaluations AS Table_1_1
                 WHERE    (ps_projectmgmt <> "na")
                 GROUP BY publisher) AS DataSource2
ON              DataSource2.publisher = DataSource1.publisher;