When I try to use the design center to insert objects, I get the following error message "This application has detected a mixed version of AEC objects participating in this operation. Newer AEC objects will be disallowed from participating in this operation."The drawings was created in acad 2012 and saved as 2007 version.The I get the error when the drawing is open and edited in 2008 version.Help!  How can I get rid of the error so that I can insert my blocks?ThanksRic

Any other software used to edit these drawings?  Sounds like you might need to download at least one object enabler.

I have downloaded the object enablers from this site, and still get the error.Is there another site with additional enablers to download?Thanks

No other software was used.

What's happening is that you have a file that has both 2007 vers compatible AEC objects and 2010 vers AEC objects.  Someone who can open 2010 version files (AutoCAD 2010-12, or 13) won't have this problem because they can interpret the newer content .  But with AutoCAD 2008 your system can't understand AEC content that may have been added by someone running the file with AutoCAD 2012, even if they saved the file back to a 2007 format you can read.
 
One way to "fix" this is to install the free version of Autodesk TrueView on your system and run the DWG conversion tool.  This should make the AEC message go away.
 
Dave.heinsite2012-08-15 04:36:04

I did as you instructed and when I opened the drawing in Autocad 2008, the proxy item dialog box did not show up when loading the drawing.However, when I tried to insert an object via the design center module, I still get the same error message.  In fact, it won't even allow me to insert the block drawings using the standard insert command.I do appreciate your help.  Any other ideas?Thanks

Then it seems the block contains AEC content from a newer version of AutoCAD.  New stuff won't work in older file formats. 
 
Can you open the block by itself?  Is it a stand-alone file?  If so run it thru the DWG convert routine also and strip the newer AEC content out.  From there you might be able to insert it into your older format file.
 
Dave.