Q: How do I use the Mac keyboard to perform the plus ( ) function?

Plus is next to delete on your keyboard (on the upper right). It is above the equals sign. Items that are displayed above other characters require you to press shift to enter them.

What happens exactly when you do 5+5=   ?

OS X has a 'keyboard viewer' that can show what keys are pressed & illustrate what shortcuts change the

behaviour…

OS X El Capitan: Use the Keyboard Viewer

http://www.macobserver.com/tmo/article/os-x-using-the-keyboard-viewer (<- has illustrations)

Enable the input menu & open the keyboard viewer, it will allow you to see what is happening as you press modifiers like shift. It may be a different combination that is required. I wonder I you have a different keyboard layout or language that is interfering somehow.

Obviously you should also check that plus works in other apps, try Text Edit.

I'm not using 10.11, so it could just be that the calculator is broken in El Capitain. I would also try the dashboard calculator, assuming you have Dashboard enabled…

http://www.gottabemobile.com/2015/10/02/how-to-enable-dashboard-os-x-el-capitan/

Try a restart.

Do a backup, using either Time Machine or a cloning program, to ensure files/data can be recovered. Two backups are better than one.

Try setting up another admin user account to see if the same problem continues. If Back-to-My Mac is selected in System Preferences, the Guest account will not work. The intent is to see if it is specific to one account or a system wide problem. This account can be deleted later.

Isolating an issue by using another user account

If the problem is still there, try booting into the Safe Mode using your normal account.  Disconnect all peripherals except those needed for the test. Shut down the computer and then power it back up after waiting 10 seconds. Immediately after hearing the startup chime, hold down the shift key and continue to hold it until the gray Apple icon and a progress bar appear. The boot up is significantly slower than normal. This will reset some caches, forces a directory check, and disables all startup and login items, among other things. When you reboot normally, the initial reboot may be slower than normal. If the system operates normally, there may be 3rd party applications which are causing a problem. Try deleting/disabling the third party applications after a restart by using the application un-installer. For each disable/delete, you will need to restart if you don't do them all at once.

Safe Mode - About El Capitan

Safe Mode Startup – El Capitan.

The calculator application uses Reverse Polish Notation by default. The RPN calculation order is number, enter, number, operation (e.g. 7 + 5 is 7, enter, 5, +). I don't have the wireless keyboard here, but on my extended keyboard, I can enter + either on the numeric keypad, or with a shift-= key. Tested on OS X 10.11.5.

VikingOSX wrote:

 

The calculator application uses Reverse Polish Notation by default.

 

Mine doesn't do that, I have to key Command R to get it to switch to RPN.