I was having this issue as well and after much frustration I finally found a solution. I use the Sewell SW-1301 which I bought with my Zietronix WBO2, this USB->Serial converter uses the Prolific chipset, there are a bunch of other converters out there that also use this chip set.
I finally found the solution at this website:
http://xbsd.nl/2011/07/pl2303-serial-us ... -lion.html
Here is the text just in case someone is looking at this years from now and that site is defunct (I hate it when that happens!). I've also included the .kext file that the site references (again in case it goes away in the future). After the included text I mention the slight differences that I had to do from the directions from the web site.
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PL2303 Serial-USB on OSX Lion
Posted on July 24, 2011 by martijn
Here’s a way to get you PL2303-based Serial-USB adapter working with OS X Lion (10.7). Based on the osx-pl2303 project on github, I’ve built a kernel extension that works with OS X Lion. You can grab the kext file here or from the link below.
Installing the kext file can be done in a few easy steps:
download and extract
cd /path/to/osx-pl2303.kext
cp -R osx-pl2303.kext /System/Library/Extensions/
next you need to fix permissions and execute bits:
cd /System/Library/Extensions
chmod -R 755 osx-pl2303.kext
chown -R root:wheel osx-pl2303.kext
cd /System/Library/Extensions
kextload ./osx-pl2303.kext
kextcache -system-cache
Good luck!
http://mpepping.home.xs4all.nl/hosting/ ... 3.kext.tgz
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My annotations:
Instead of using the terminal window to do the copy (cp -R osx-pl2303.kext /System/Library/Extensions/) I just opened the Extensions folder in the finder and dragged in the osx-pl2303.kext file into it. If you don't know how to find the Extensions directory then do this, open the Finder window and look at the devices section on the left of the window, your Mac should be there. Click on that and then in the right side of the window you should your hard drive, if you didn't rename it then it should be "Macintosh HD". Click on that, then click on "System", then "Library", then "Extensions". That is where you drop the file.
For the next part you do need the Terminal window so open it and do as the original procedure states, the change here is that I had to preface the commands with "sudo", so you would do this instead:
cd /System/Library/Extensions
sudo chmod -R 755 osx-pl2303.kext
sudo chown -R root:wheel osx-pl2303.kext
cd /System/Library/Extensions
sudo kextload ./osx-pl2303.kext
sudo kextcache -system-cache
During some of these commands it will more than likely ask you for your password, this is normal.
After I did this, I then rebooted the Mac and was able to connect TunerStudio up to my Megasquirt box in the connection settings with the "/dev/cu.PL2303-000012FD" port selection.
Hopes this helps someone else out with a lot less frustration than I had! Everything else on the Mac is so much better, why was this so hard?
Kevin