try without user::. just use your variable name.

Public Overrides Sub Input0_ProcessInputRow(ByVal Row As Input0Buffer)

'Initialise the variable

Dim vars As IDTSVariables90

'Lock a variable so that we can write to it

Me.VariableDispenser.LockOneForWrite("varFoo", vars)

'Write the value to the variable

vars(0).Value = "Foo"

'Release the lock

vars.Unlock()

End Sub

hope this helps