Can`t make Arduino Uno R3 work under Mac OSX 10.10.3

Hello!

I`m new in this forum and in working with arduino technology in general.

I bought a Arduino Uno Starter Kit, downloaded the latest software (it was no driver installation necessary), then I connectet the board via usb with my MacBookPro. The Power LED is green, the "L" LED blinks orange. I can find the board under Hardware/USB (pic 1). But if I start the arduino app i only find /dev/cu.usbmodem1421 and not dev/tty.usbmodem1421 as I am supposed to according to my manual.

I tried changing the usb cable. i didnt work.

Can you help me?

Thank you.

Bildschirmfoto 2015-06-18 um 16.48.24.jpg