Macbook Pro does not recognize bootable USB drive
The Problem is most likely that the USB Drive was not properly formated and is missing the EFI Partition. You could check via the Terminal by using 'diskutil':

diskutil list


The Result is a list of all Drives and their partions. A bootable USB Drive should have a separate partition like this:

/dev/disk11 (external, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *16.0 GB    disk11
   1:                        EFI EFI                     209.7 MB   disk11s1
   2:                  Apple_HFS OS X 10.11 Installat... 15.7 GB    disk11s2


The easiest solution would be using a third party tool. You should try again using the Freeware 'DiskMaker X'. It formats and creates a bootable USB Drive with the OSX-Installer in a very easy and reliable manner.