Note: Between the steps below you can check if your problem is fixed in Excel 2011.
Shut Down and Restart your Mac
But sure that you have installed the latest updates for your Mac and Office 2011.
Installed add-ins can also give problems so go to Tools>Add-ins and uncheck them all in the add-in list and Close/Reopen Excel. If you not have problems now put them back one at the time (Close/Reopen Excel each time before you test) untill you find the one that give you problems.
Click on the Spotlight icon in the top right corner of your Mac and enter "Disk Utility"to find and open open this program. Select your harddrive and click on "Repair Disk Permissions" and when it is ready click on "Verify Disk Permissions".
Problems with Preference and Toolbar files in Office 2011.
But where are those files on your Mac ?
In the newest OSX versions the folders with the files are hidden
1: Close all Office programs first
2: Open a Finder window
3: Hold down the Alt key when you press on Go in the menu and click on Library
In the com.microsoft.Excel.plist file Excel saves the changes you make to your preferences. If you have never made a change to your preferences, it is possible you will not have this file.
Application Support:Microsoft:Office:Preferences:Office 2011 folder
If you have ever made changes to your menu or toolbars you will find a file here named
Microsoft Excel Toolbars
4: The best way to discover whether the files above are giving you problems on your Mac is to drag the ones you find to your Desktop, and then re-start Excel. If the problems are fixed you should delete the files from your desktop. Excel will create new ones for you as soon as you change things in preferences or in your menus/toolbars.
Fonts can give many problems on a Mac, check out the webpages below for more information.
Create a new user account, see if Excel work correct in this new user account.
If nothing is working I suggest : Complete removal followed by a fresh install. Follow the steps in this article : http://support.microsoft.com/kb/2398768
Start with step 1 and 3 and then reboot your Mac. If it is not working after the reboot do step1, 3 and 7 and reboot (you have to repeat step 3 each time).
See also the information on this page : http://www.officeformachelp.com/office/install/remove-office/
Uninstall Office 2016 for Mac :
In this article you can also find where Office 2016 store his preference files.