The CASE is a function. The way you used it doesn't make any sense.
Try

ON (Table1.Fld1='' AND Table1.Fld2 = Table2.Fld2 )
   OR 
   (Table1.Fld1<>'' AND Table1.Fld1 = Table2.Fld1)

>
>I am trying to "translate" the VFP code that has IIF() in the JOIN into SQL Server systax. Here is what I have (but gives me error):
>
>

>select * from Table1 left join Table2 ON CASE WHEN LEN(RTRIM(Table1.Fld1))=0 THEN Table1.Fld2 = Table2.Fld2 
>  ELSE Table1.Fld1 = Table2.Fld1 END
>

>
>What am I missing? TIA