After successful UNZIP of the file AutoOnPageLoadPopup_UNZIP. You will be able to see the Documentation folder and two different version of the extensions. One is for Joomla 1.5.x to 2.5.x and the other for 3.x. Choose the correct file to install. Installtion of incorrect file may give you errors.
- For Joomla 1.5.x to 2.5.x install : mod_onpageload_popup2.5_vX.x.zip
- For Joomla 3.x install : mod_onpageload_popup3..x_vX.x.zip
Go to Extensions > Extension Manager and click on the Upload Package File Tab.
Click on browse to choose the correct installtion file and click on "Upload and Install"