I bought cd from costco and i couldn't download. please help

Before you download the CD, be sure you create a Tubo Tax Intuit account.  go to InstallTurboTax.com. If you don't have an Intuit account, you will create one. Then download the CD. AFTER!  You will then select Windows or MAC.  

Even if you buy the CD you should use the download method to install the program.  Then you can for sure download it again like to reinstall or to install on another computer.  You might have trouble installing the CD again or getting the download after using the CD.  

How to install with a License Code

https://ttlc.intuit.com/community/tax-topics/help/activating-turbotax-cd-download-software/00/852971

There should be instructions with the CD on how to download it instead.  In fact this link says you can't use both the CD and download. 

How to download Turbo Tax instead of using the CD

https://ttlc.intuit.com/community/installing/help/i-bought-the-cd-but-my-computer-doesn-t-have-a-cd-drive/00/26450