1. Download Notepad++: http://www.notepadplusplus.org
2. Install Notepad++
3. Download file RouterOS Syntax-Highlighter and extract the notepad++ xml file within.
4. Open Notepad++
5. Click Language > User-Defined > click Import button.
Import the xml file you unzipped.
6. Close and reopen Notepad++
7a. Open an .rsc script, and syntax highlighting will be visible.
7b. Alternatively open any file and select the “RouterOS” language option

Thanks to dtoffo on the MikroTik forums for the original release of this highlighter.