To use the Ubuntu installed in your Mac you have to install rEFInd which is used to manage boot loaders on a Mac(The rEFInd will auto-detect the installed boot loaders .


   **How to use rEFInd**

 - Download rEFInd from [here][1]*
 - Open a Terminal window by pressing Command + Space and, typing Terminal, and pressing Enter.
 - Drag and drop the install.sh file from the downloaded zip file into the terminal window and press Enter to run it.

 - Shut down your Mac -a full shut down, not a restart — and boot it back up again. You should see the rEFInd boot manager screen.(If not Press and Hold Alt button and when it boots up select EFI Disk Icon(If Exist)).

*Now for your next Question on how to uninstall Ubuntu from Mac the best solution I suggest is:*

 - Get an Ubuntu – Live CD. You will need it to delete the Linux
   partitions Boot into the Ubuntu Live CD (press the C button while
   restarting with the Ubuntu CD/DVD in the drive) 
 - Start Gparted the
   Ubuntu partitioning program. If unsure on how to do this, just go the
   the ‘Dash Home’ and search for Gparted Delete the Linux partition Set
   the swap partition to “off”. 
 - Do NOT re-size the Mac-OS partition while in
   Gparted this will be done from Mac-OS Reboot the laptop and boot into
   Mac OS X Start the Disk Utility application and go to ‘Partition’
 - Re-size the Mac OS partition by dragging it to the bottom to take up
   the space that had been taken up by Ubuntu Re-boot and make sure
   those changes have taken effect Next we need to delete rEFInd 
 -  Instructions for
   uninstalling rEFInd are located at their site here In my case method
   1 was the one applicable Delete the efi folder Delete rEFInd
   folder in “Library/StartupItems” Reboot and that’s it!!

***Edit:***

    error "Could not set boot device property: 0xe00002bc"

*If you get some errors like mentioned above it is because System Integrity Protection is enabled on your Mac.You can disable it by following the steps below.*

**How to Disable SIP** 

 - Restart the computer, while booting hold down Command-R to boot into
   recovery mode.

 - Once booted, navigate to the “Utilities > Terminal” in the top menu
   bar.

 - Enter csrutil disable in the terminal window and hit the return key.

 - Restart the machine and System Integrity Protection will now be
   disabled.

Credits: [HarlemSquirrel][2] 


  [1]: http://sourceforge.net/projects/refind/
  [2]: https://apple.stackexchange.com/users/149628/harlemsquirrel