1) Open a blank drawing.
2) Type "Menuload"
3) Browse > C:\Users\*?????*\AppData\Roaming\Autodesk\MEP 2011\enu\Support
* = a directory with a company name, employee number, etc...that is unique to your machine.
4) I believe the file you are looking for is "acad.CUIX"
Give this a shot....
I'll answer KBrown's question first which leads into the second answer.
The desktop icon not only switches the profile, but launches the program into that profile. This means that it switches the template. Templates are everything in AutoCAD MEP. Use the wrong one and run into all sorts of problems. Ever try launching MEP, and switching profiles from the Options menu? It will change the profile, but you have a drawing file open, which originated out of the first profile. There is nothing in the program that will close that already open file for you. You have to remember to close that file, and then start a new file using a template intended to be used by the new profile you just switched over to in the Options menu. SO. WE USE DESKTOP ICONS. The desktop icons always guarantee that you are using a drawing file started from a template that matches the profile. We would not want to use an Architectural template in MEP, or vice-versa, or a Core AutoCAD template. Features break when you do that. Our templates have settings that do not exist in the other templates. Then those settings all have to be set right. Think Part Routing Preferences and System Definitions. We have other items in our template like blocks that support some styles (Ex. Rise-Drop Blocks), and layers that support some display components (Ex. Hidden Lines). Then there are the Displays themselves. All are required to draw duct and all other objects. Features also break when you xref an MEP file into a Core AutoCAD file. The Core AutoCAD file does not contain our displays needed to display our MEP objects properly, so colors change and pipe may go from 1-line to 2-line.
The second (original) question: How to find and switch over to the Classic workspace. That workspace is only found in Core AutoCAD. When you first install AutoCAD MEP, there is a customization screen for Content Packs. There is a content pack that allows us to add a desktop icon for Core AutoCAD. This option is only available when you first install the program. It is not available from the Add/Remove Programs > Add Features, after installation. If you install this desktop icon, it is named "AutoCAD MEP As AutoCAD." Use this desktop icon and it launches Core AutoCAD. Once you have Core AutoCAD open, then you can use the Workspace selector to change over to the Classic workspace. The Classic workspace does not work inside AutoCAD MEP or Architecture. If you want toolbars in AutoCAD MEP, you can transfer them in using the CUI editor. The last time Autodesk made toolbars was in 2009. So you need to find the 2009 CUI files to transfer them over. For your convenience, the AutoCAD MEP 2009 CUI files with toolbars are attached. We do not have toolbars for new features introduced since 2009. You can make buttons for them in the CUI editor.
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Hope this helps.