Hello,
After the update to El Capitan, I can't see the serial port for my nano clones, CH340 equiped 
I tried this command:
sudo nvram boot-args="kext-dev-mode=1"
without success, doesn't work with El capitan released (but seems ok with Yosemite and El capitan beta versions).
I found this solution (https://www.kiwi-electronics.eu/blog?journal_blog_post_id=7):
Just like Yosemite, El Capitan requires kext driver signing. How this can be disabled in OSX 10.11 is changed however.
To get the CH340 drivers to work in El Capitan you need to use the new tool csrutil as follows:
Reboot and press CMD+R immediately after hearing the startup sound to boot to Recovery Mode
Open Terminal
Execute the following command: csrutil enable --without kext
Reboot
Just another link with the same workaround:
Claudio