I still don\'t know how to add the installdir into the PATH of the Windows System Variables after I went through the WIX tutorial.
I tried to use
           
        Another thing to note is, Environment need to be placed inside a component with directory, e.g.
<DirectoryRef Id="TARGETDIR">
  <Component Id="Path" Guid="{xxx-xxx-xxx-xxx}">
    <Environment Id="PATH" Name="PATH" Value="[INSTALLDIR]" Permanent="no" Part="last" Action="set" System="no" />
  </Component>
</DirectoryRef>
Details of Wix Element described at Environment Element
Had the same exact problem, this have worked for me:
        <Directory Id="TARGETDIR" Name="SourceDir">
            <Directory Id="ProgramFilesFolder">
                <Directory Id="INSTALLFOLDER" Name="DataBaseds_Service_Installer" />
            </Directory>
        </Directory>
    <ComponentGroup Id="Components" Directory="INSTALLFOLDER">
      ...some components  
    </ComponentGroup>
    <DirectoryRef Id="TARGETDIR">
      <Component Id="MYSQL_PASSWORD" Guid="..."
        <Environment Id=HERE YOU CAN ADD THIS  :)/>
      </Component>      
    </DirectoryRef>
You should be able to use:
<Environment Id="PATH" Name="PATH" Value="[INSTALLDIR]" Permanent="yes" Part="last" Action="set" System="yes" />
This should add a new entry to the environment path, set to [INSTALLDIR].