user4931: Problem
I have an Arduino Nano connected via a USB cable (Type A to Mini Type B) to my MacBook Air (Mid 2013 Model). The PWR LED is on while the arduino is connected. 
Opening the Arduino software, pulling down the Tools > Serial Port menu does not show me a valid serial port. All I see is:
/dev/tty.Bluetooth-Incoming-Port
/dev/cu.Bluetooth-Incoming-Port
/dev/tty.Bluetooth-Modem
/dev/cu.Bluetooth-Modem

What I've tried and did not work

I have installed the correct FTDI driver (v2.2.18). x64 for my MacBook Air's Corei5, file FTDIUSBSerialDriver_10_4_10_5_10_6_10_7.mpkg. Restarted after installation.
Tried connecting arduino to both USB ports on my Air.

More Info

Running Mac OSX 10.10 Yosemite
Same arduino and cable work when connected to a Windows machine or a Raspberry Pi.

If it helps, I can see the USB Serial in my Apple Logo > About This Mac > System Report under Hardware > USB
USB2.0-Serial:

  Product ID:               0x7523
  Vendor ID:                0x1a86
  Version:                  2.54
  Speed:                    Up to 12 Mb/sec
  Location ID:              0x14100000 / 8
  Current Available (mA):   500
  Current Required (mA):    Unknown (Device has not been configured)

I'm out of ideas and options. Any help would be greatly appreciated.
Thanks in advance :)

user4931: This solved it for me.

Download this driver
Install it
Run sudo nvram boot-args="kext-dev-mode=1"
Reboot

Serial ports now showed up in the Arduino IDE and also when I used ls -1 /dev/tty*
Hopes this helps someone.
Reference: This thread

4931: <h1>Problem</h1>

<p>I have an Arduino Nano connected via a USB cable (Type A to Mini Type B) to my MacBook Air (Mid 2013 Model). The PWR LED is on while the arduino is connected. </p>

<p>Opening the Arduino software, pulling down the <code>Tools &gt; Serial Port</code> menu does not show me a valid serial port. All I see is:</p>

<pre><code>/dev/tty.Bluetooth-Incoming-Port
/dev/cu.Bluetooth-Incoming-Port
/dev/tty.Bluetooth-Modem
/dev/cu.Bluetooth-Modem
</code></pre>

<h2>What I've tried and did not work</h2>

<ul>
<li><p>I have installed the correct FTDI driver (v2.2.18). x64 for my MacBook Air's Corei5, file <code>FTDIUSBSerialDriver_10_4_10_5_10_6_10_7.mpkg</code>. Restarted after installation.</p></li>
<li><p>Tried connecting arduino to both USB ports on my Air.</p></li>
</ul>

<h2>More Info</h2>

<ul>
<li>Running Mac OSX 10.10 Yosemite</li>
<li>Same arduino and cable work when connected to a Windows machine or a Raspberry Pi.</li>
</ul>

<p>If it helps, I can see the USB Serial in my <code>Apple Logo &gt; About This Mac &gt; System Report</code> under <code>Hardware &gt; USB</code></p>

<pre><code>USB2.0-Serial:

  Product ID:               0x7523
  Vendor ID:                0x1a86
  Version:                  2.54
  Speed:                    Up to 12 Mb/sec
  Location ID:              0x14100000 / 8
  Current Available (mA):   500
  Current Required (mA):    Unknown (Device has not been configured)
</code></pre>

<p>I'm out of ideas and options. Any help would be greatly appreciated.</p>

<p>Thanks in advance :)</p>
 4931: <p>This solved it for me.</p>

<ul>
<li>Download <a href="http://www.wch.cn/downloads.php?name=pro&amp;proid=178" rel="nofollow">this driver</a></li>
<li>Install it</li>
<li>Run <code>sudo nvram boot-args="kext-dev-mode=1"</code></li>
<li>Reboot</li>
</ul>

<p>Serial ports now showed up in the Arduino IDE and also when I used <code>ls -1 /dev/tty*</code></p>

<p>Hopes this helps someone.</p>

<p>Reference: <a href="http://forum.arduino.cc/index.php?topic=261375.0" rel="nofollow">This thread</a></p>
