If you see this message "zynga poker security alert"after opening texas holdem poker on facebook, that means you tried to acess zynga poker from a different country(maybe you maybe someone else...).

You should go to your email and check your inbox messages.

On your inbox you will see an email from zynga texas holdem poker, open it

If you entered from this country then click on "authorize play from [country]"

If you didnt, click on "do not authorize" and change your facebook password