Legacy Mode in Joomla 1.5 is needed in order to install Joomla 1.0.* extensions.
Step 1: Login to administrator’s panel
Step 2: Step 1: Go to Extensions and then Plugin manager.
Step 3: There find ” System – Legacy” and check it
Step 4: Click on “Enable” from the top right menu.
Note : The above instructions are valid for the stable Joomla 1.5 versions. If you are using Joomla 1.5 beta versions, you can enable the “Legacy mode” in the Global Configuration.