Serato DJ 1.9.3 now has the latest OS X 10.11 El Capitan drivers for the RANE SL2, SL3, and SL4 products bundled. This means that launching Serato DJ 1.9.3 with your SL2, SL3, or SL4 connected - will cause it to prompt you to install the latest driver.
After installing the driver and restarting your computer as instructed, you may find Serato DJ does not detect your hardware.

Work-around;

To resolve this issue, simply hot-plug (disconnect and reconnect) your SL hardware after your computer has restarted, and the new driver should be correctly recognised. This only needs to be done the first time after installing the driver.

If you have any issues after doing this, remember to check the Troubleshooting Connection Issues on OS X guide, or Rane's El Capitan Troubleshooting guide.