Windows

World of Goo uses standard OutputDebugString calls, so you can view them with a utility like DbgView.

Linux

The game creates a logfile in $HOME/.WorldOfGoo/WorldOfGoo.log. Note that this is overwritten each time you run the game.

Mac

Unknown?

Easy answer: Use the new strings file available in goomod version 1.1.

Please use the above method for new addins. For historical interest (and in case you need to merge a different file), the old answer is below:

Create a file in merge/properties/ called text.xml.xsl.

Inside this file put something along these lines:

<xsl:transform version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">

 <!-- Copy everything not matched by another rule -->
 <xsl:template match="* | comment()">
  <xsl:copy>
   <xsl:copy-of select="@*"/>
   <xsl:apply-templates/>
  </xsl:copy>
 </xsl:template>

 <!-- Append our strings to the end -->
 <xsl:template match="/strings">
  <xsl:copy>
   <xsl:copy-of select="@*"/>
   <xsl:apply-templates/>

    <string id="SIGNPOST_JINGLEBALLS_1"
            text="Hello, I am a Sign.|This is page 2.|-the Sign Painter" />

  </xsl:copy>

 </xsl:template>
</xsl:transform>

Notice that the | symbol separates the screens of text.

For more information, see the page about the merge directory.

Distributing your addin as a goomod file:

• allows the user to easily install it with GooTool without needing to run an exe or extract a zip in the right place
• allows the user to easily uninstall it, ensuring that no leftover files remain
• helps ensures your addin does not mess with future 2dboy official updates
• helps prevent conflicts between your addin and others
• allows your addin to be compatible across all platforms, as GooTool takes care of the difference in encryption and PNG file formats
• allows your addin to be distributed through this site
• allows your addin to be automatically downloaded in GooTool (coming soon)

See also: How can I create my own addin?

Check out the addin file format page. For the moment you will need to create a Zip file of the necessary structure by hand and rename it to have a goomod file extension. It is anticipated that a future release of GooTool will have features to assist in addin development.