To get this to work right, you're gonna have to edit your eq2_ui.xml file, it's easy

NOTE: if you want to install the classic chat window thing, skip to the end

Step 1:
Copy eq2ui.xml from the UI/Default folder to what ever folder you're installing this UI piece to
NOTE: skip this step if there is already a eq2ui.xml file in the folder you're installing to.

Stop 2:
Open eq2ui.xml in a text editor (like note pad)
<?xml version="1.0" encoding="utf-8"?>
<Page AbsorbsInput="false" Cursor="default" CursorSet="default_set" IgnoreTab="false" MinimumSize="800,600" Name="root" PackSize="1,1" PackSizeProp="0002/0003,0002/0003" ScrollExtent="1024,768" Size="1024,768" TooltipStyle="/TooltipStyles.tooltip_default">
<include>eq2ui_skininfo.xml</include>
<include>eq2ui_gamedata.xml</include>
<include>eq2ui_commonelements.xml</include>
<include>eq2ui_textstyles.xml</include>
<include>eq2ui_shadowstylesnew.xml</include>
<include>eq2ui_specialelements.xml</include>
...
...
<include>eq2ui_community.xml</include>
<include>eq2ui_examine.xml</include>
<include>eq2ui_mainhud.xml</include>
<include>eq2ui_proxyactor.xml</include>
<include>eq2ui_ime.xml</include>
</Page>
Although the exact lines may differ.
and add this the following line somewhere amungst the rest of the includes
<include>PUI_styles.xml</include>

Step 3:
save eq2ui.xml to the folder you're installing this UI piece to.


Installing eq2ui_mainhud_mainchat_classic.xml
Step 1:
you don't need to edit eq2ui.xml to install this file, since all the scroll bars 
and such are the default ones.  You still can, but it's not needed.

Step 2:
rename the file
eq2ui_mainhud_mainchat_classic.xml
to 
eq2ui_mainhud_mainchat.xml

that's it.