Listbox multiselect- VBA code error

I'm a bit of a beginner at using visual basics to code. I have inserted an activex listbox into my sheet. Its is multiselect. I want the selectiong to be inserted into a single cell on a seperate sheet. I have found the code to do it and had it working on my Excel 2007 at home but i opened it at work on Excel 2010 and I get the error 'Compule error: cannot find project or library"
I can't work out what the error is. this is the code I have written:

Private Sub ListBox1_LostFocus()

Dim listItems As String, i As Long

With ListBox1

For i = 0 To .ListCount - 1

If .Selected(i) Then listItems = listItems & .List(i) & ""

Next i

End With

If Len(listItems) > 0 Then

Sheets("English IEP").Range("C7") = Left(listItems, Len(listItems) - 1)

Else

Sheets("English IEP").Range("C7") = ""

End If

End Sub

Would anyone be able to point out where I have gone wrong.
Thanks so much