Arduino not detected On El Capitan Public Beta 6
As noted elsewhere, there have been plenty of problems with the CH340 driver and the OSX El Capitan betas, culminating into the driver flat out not working in the latest beta. This is either a bug with OSX or a bug in the driver. I would lean towards this being a bug in the driver, which hasn't been updated since late 2013.

There are other 3rd party drivers that might work on El Capitan. mac-usb-serial.com is the most reliable looking one that I could find, and it does appear to be actively developed. It does cost money, but it is signed (which the mfg. driver isn't) and it has a test utility you can use before purchasing.

The only other options after that are:


Hope that WCH update their driver (don't hold your breath).
Program using an external programmer
Buy an Arduino not using the CH340