Usa la proprietà text della combo :
codice:
Option Explicit

Private Sub Command1_Click()
    myCombo.Text = "Limone"
End Sub

Private Sub Form_Load()
    With myCombo
        .AddItem "Arancia", 0
        .AddItem "Limone", 1
        .AddItem "Mela", 2
    End With
End Sub

Private Sub myCombo_Click()
    MsgBox myCombo.ListIndex
End Sub
Ciao,