Legacy Mode:

Legacy mode in Joomla 1.5.x is required to install Joomla 1.0.x plugins, and extension.

Instructions:

You can enable  legacy mode using following steps;

Step 1: Login to Joomla Administrator Section

Step 2: Click on Extensions > Plugin Manager

Step 3: Look for "System - Legacy", check off the check box and then click on "Enable" from the top right menu.

Note:

Above mentioned instructions are only valid for the stable Joomla 1.5.x versions. If you are using Joomla 1.5.x beta versions, you can enable the "Legacy mode" in the Global Configuration section.