Quickest, easiest (and laziest way)
In a code module 

Sub Auto_Close
    Application.Calculation = xlAutomatic
End Sub
Sub Auto_Open
    Application.Calculation = xlAutomatic
End Sub
If it's still switching (which it may) you need to search for the vba code which switches it to manual
1. Switch to VBEIDE (VBA editor)
2. Select a module
3. CTRL-F......................(find)
4. Application.Calculation
5. Select "Entire Project"

......... Step through each instance looking for the key word manual... Where you find one - ensure it has Application.Calculation = xlAutomatic somewhere near the End Sub.