Which reference are you trying to Add? You can check the references like this:
Dim r As Reference, result as boolean
result = false
For Each r In Application.References
If r.Name = "reference your looking for" Then
result = true
End If
Next
If result is false after this, you can use the references.addfromfile statement to add the reference, you will need the full path to the reference you wish to add though...
Hope this helps