Open an Excel File from within a macro in Excel 2007 from user input

Maybe:

Sub NewRackinCO() 'WarrenSmiley
' New Rack in CO
Sheets("New Rack in C.O.").Activate
' This button allows you to add a new rack into a CO
'
'   Dimension the Variables
    Dim Cust As String
    Dim SiteName As String
    Dim RackPosNo As String
    Dim ACDCVolt As String
    Dim strPath As String
'   Stores Input data as variables
    Cust = Range("B3")
    SiteName = Range("B4")
    RackPosNo = Range("B5")
    ACDCVolt = Range("B6")
    strPath = Range("A21")

'   Opens the workbook "SiteName" in the "Cust" directory to add a new rack
    Workbooks.Open FileName:=strPath
End Sub