Create a new file with the following contents and save it as haxe.plist

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
    <key>keywords</key>
    <array>
        <string>break</string>
        <string>case</string>
        <string>catch</string>
        <string>continue</string>
        <string>default</string>
        <string>do</string>
        <string>else</string>
        <string>for</string>
        <string>if</string>
        <string>in</string>
        <string>return</string>
        <string>switch</string>
        <string>throw</string>
        <string>try</string>
        <string>while</string>
        <string>class</string>
        <string>enum</string>
        <string>extends</string>
        <string>function</string>
        <string>implements</string>
        <string>import</string>
        <string>interface</string>
        <string>inline</string>
        <string>override</string>
        <string>package</string>
        <string>private</string>
        <string>public</string>
        <string>static</string>
        <string>typedef</string>
        <string>var</string>
        <string>ArrayAccess</string>
        <string>Array</string>
        <string>Bool</string>
        <string>Dynamic</string>
        <string>Float</string>
        <string>Int</string>
        <string>Iterator</string>
        <string>Iterable</string>
        <string>Null</string>
        <string>Object</string>
        <string>String</string>
        <string>UInt</string>
        <string>Void</string>
        <string>cast</string>
        <string>new</string>
        <string>super</string>
        <string>this</string>
        <string>trace</string>
        <string>typeof</string>
        <string>untyped</string>
        <string>false</string>
        <string>null</string>
        <string>true</string>
        <string>undefined</string>
    </array>
    <key>beginCommand</key>
    <string></string>
    <key>endCommand</key>
    <string></string>
    <key>beginInstruction</key>
    <string></string>
    <key>endInstruction</key>
    <string></string>
    <key>firstString</key>
    <string>&quot;</string>
    <key>secondString</key>
    <string>&apos;</string>
    <key>firstSingleLineComment</key>
    <string>//</string>
    <key>secondSingleLineComment</key>
    <string></string>
    <key>beginFirstMultiLineComment</key>
    <string>/*</string>
    <key>endFirstMultiLineComment</key>
    <string>*/</string>
    <key>functionDefinition</key>
    <string>^\s*function\s+.*\n?\s*{</string>
    <key>removeFromFunction</key>
    <string>function </string>
    <key>keywordsCaseSensitive</key>
    <true/>
    <key>recolourKeywordIfAlreadyColoured</key>
    <true/>
    <key>autocompleteWords</key>
    <array/>
</dict>
</plist>

Put the file in your Home Directory/Library/Application Support/Fraise (it probably won't exist if this is the first time you do this). Edit or create a new plist file named SyntaxDefinitions.plist. If the file already exists, use the existing format to add a dictionary with the keys:

name (String) = Haxe
file (String) = haxe
extensions (String) = hx

Otherwise if the file doesn't exist, its easier to just create a new one and put the following contents:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<array>
    <dict>
        <key>name</key>
        <string>Haxe</string>
        <key>file</key>
        <string>haxe</string>
        <key>extensions</key>
        <string>hx</string>
    </dict>
</array>
</plist>

Restart Fraise and make sure the Haxe definition appears at Preferences -> Advanced -> Syntax Definitions (it will probably be the last option).