By default, only processes that are run directly by root are allowed to load kexts. When a user (non-root) process tries to load a kext, an error is logged in system.log similar to:

Request from non-root process 'netbiosd' (euid 222) to load /System/Library/Extensions/smbfs.kext - not allowed.

You can resolve the issue by allowing the specific kext (e.g.: smbfs.kext) to be loaded by non-root processes. This is done by editing the Info.plist file that's included with the kext, usually at /path/to/kext-name.kext/Contents/Info.plist.

1. Open a Terminal.
2. Locate the kext and edit its Info.plist file.
   In our example, this would be the file: /System/Library/Extensions/smbfs.kext/Contents/Info.plist.

   sudo nano -w /System/Library/Extensions/smbfs.kext/Contents/Info.plist

3. Add the following lines at the bottom of the file, right above the closing </dict>:

   <key>OSBundleAllowUserLoad</key>
   <true/>

   (To save in nano, hit Ctrl + O, then press Enter.)
4. After the edit, reboot OSX for the change to take effect.