There is a program called EFIStudio which would generate the EFI string after you select the card that you have from the list (within the application). Then once you click on Add Device, it will generate a string (known as hex string) for you and it will give you an option to save it directly in the com.apple.boot file. Once its saved, remove the injector file (I assume in your case its Natit.kext) and restart the machine.
Also before you save it to the com.apple.boot file, make sure you make a copy of the file in your root drive. Its located in /Library/Preferences/SystemConfiguration. Just incase if the string doesn't work, you can revert back to the original one.
just to add on to this... you can also use pcwiz's OSx86 Tools Utility (http://pcwizcomputer.com/index.php?option=com_content&task=view&id=91&Itemid=48) to do this as well. You want to add the string in the com.apple.Boot.plist file right at the end before the </dict>
</plist>
in the end, your .plist file should look something like:
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple Computer//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
<key>Kernel</key>
<string>(the name of the default kernel you use)</string>
<key>Kernel Flags</key>
<string>(whatever boot flags you need or want to always use without typing in at every boot)</string>
<key>Timeout</key>
<string>(whatever value in seconds you want)</string>
<key>device-properties</key>
<string>**YOUR-EFI-STRING-HERE**</string>
</dict>
</plist>