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

As part of a project I am working on I take user input from list boxes, concatenate the input to create the path and file name to open and store that in a cell. I now am trying to open that file as part of a macro. Cell A21 contains this: =CONCATENATE("J:\Information\Rack Power Database\Programming\Deployed Racks\", B3, "\", B4, ".xlsm") . Any suggestions as to how to best accomplish this as I keep erroring out on the last line of code? Not sure if this needs to be identified as a value or if it should not be dimensioned as a string.

Sub NewRackinCO()
' New Rack in CO
' 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
    Sheets("New Rack in C.O.").Select
    Range("B3").Select
    Cust = ("B3")
    SiteName = ("B4")
    RackPosNo = ("B5")
    ACDCVolt = ("B6")
    strPath = ("A21")
'   Opens the workbook "SiteName" in the "Cust" directory to add a new rack
    Workbooks.Open Filename:=strPath
End Sub

Using Debug, step through your code, one line at a time, and stop just before the last line it crashes on. Put your cursor over the strPath variable to see the current value for that variable. I suspect it won't be quite what you think it should be, revealing your error.