To use the jedit syntax highlighting install qc.xml to the syntax
directory for jedit

# For Windows Users that directory is
C:\Users\username\.jedit\modes

# For Linux users that directory is
/home/username/.jedit/modes

# For Mac users that directory is
/Users/username/Library/jEdit/modes

After the file is installed, a mode line needs to be added to
a file caled catalog in that same directory.

Add the following line:
<MODE NAME="QuakeC Code" FILE="qc.xml" FILE_NAME_GLOB="*.qc" />

inside the <MODES> block before the end tag </MODES>. If the file
does not exist, you can simply make one and use the following:

<?xml version="1.0"?>
<!DOCTYPE MODES SYSTEM "catalog.dtd">
<MODES>
    <MODE NAME="QuakeC Code" FILE="qc.xml" FILE_NAME_GLOB="*.qc" />
</MODES>