This error is as a result of Microsoft security update MS27-027 released May 2012.
Unfortunately this update does not correctly update certain Office dll files, mainly MSCOMCTL.OCX which is used by Excel to display controls on the forms, for example the treeview control used to display the list of Entities.
Delete all *.exd files on the C:\Drive
1. Open a command prompt (DOS Window)
2. Navigate to the Documents and Settings folder C:\Documents and Settings> (C:\Users on Windows 7)
3. Enter the following: C:\Documents and Settings> del/S/A:H/A:-H *.exd (and press enter)
Alternatively manually locate and delete the following:
a) C:\ Documents and Settings\\Application Data\Microsoft\Data Forms\*.exd
b) C:\Documents and settings\username\AppData\Loca\\Temp|VBE\*.exd
If this does not fix the issue please contact firstname.lastname@example.org