You can download CodeIgniter free to your system from official website of CodeIgniter or follow the images i posted.
>> Click the link to start download http://codeigniter.com
>> Click on yellow round circle to start download.
>> The save as option will appear checked the Save File and click on OK.
>> Select the location to save the file in your system.
>> CodeIgniter 1.7.2.zip is downloading in your system.
>> Download is completed.
>> CodeIgniter Direct Download click here.