[ShadowChemosh]

So as Toby said its not something that you can do in the editor. The RAW XML is provided to everyone under the folder 'Source' so adding a new one is not that hard. You can add the following to any existing .user file just before the </document> using a text editor.

I added a Pre-Req validation that says this FE is only valid if you have the variant class Arcane Hunter. In addition this only works for those that have the community stuff as Arcane Hunter does not exist in d20 core.

  <thing
    id="feS2Arcani"
    name="Arcanists"
    compset="FavEnemy"
    isunique="yes"
    description="Your favored enemy bonus works against those that cast Arcane spells.">
    <exprreq message="Arcane Hunter variant class required."><![CDATA[
      tagis[ClassVary.cvArcHunte] <> 0
      ]]></exprreq>
    </thing>

If not comfortable editing XML files make sure you make backup copies before trying. Once added then restart HL or do a CTRL-R to recompile everything.