Locating Autorecover files Excel 2011 doesn't find

Found 'em! Here are the solution steps:

1. Go to ~\Library\Application Support\Microsoft\Office\Office 2011 AutoRecovery in Finder (on OSX10.7+ this may first require making the Library directory visible using one of the methods described here or here)
2. Change the Extension of the Excel autorecover files of interest from .xlsx to .xlsb
3. Open the files with Excel (either double click or using Excel's File>Open menu)
4. Enjoy!

Discussion:

As described in this post, if you are lucky, a newer auto-recovery version can often be found in ~\Library\Application Support\Microsoft\Office\Office 2011 AutoRecovery. Files in this directory often have somewhat munged/cryptic names, but you can always open them up to find the best one.

However, in the current version of Excel for Mac 2011 (14.3.1, maybe others) there is a bug that prevents easily opening these backup files. Specifically, the filenames are listed with an .xlsx extension (corresponding to Excel XML format), but are actually in a different format. As a result attempting to open them in Finder or through the Open menu fails, claiming the files are corrupt. The exact error is

Microsoft cannot open this file The file format or file extension is not valid. Verify that the file has not been corrupted and that the file extension matches the format of the file.

Changing the file extension to .xlsb (Excel binary format) or .xlk (older Excel backup) will allow Excel to open the file.

Bryan P's answer is great. Agreed 100%. I just wanted to add one other trick to try if you can find the files but can't open them. That suggestion: OpenOffice.

Excel could not open its own auto-recover file no matter what extension I used. However, OpenOffice had no problem.

The only twist is that you'll need to drag the file onto OpenOffice's icon in the dock or right click to open with OpenOffice, because you're likely to be unable to navigate to the file through the dialogue windows.