You go to your Account page, and in the Contact Info section, click on Change. Then click on Change Mailing Address. You will then be allowed to change the country of your residence.
I believe you will not be allowed to "represent Venezuela" once you submit a US address. You will probably have to switch to the US flag. This is the reason why I myself decided not to circumvent WGT's blocking of tournaments for most countries, although I have numerous relatives living in Germany and submitting their street address would have been no problem. But, I want to see the Slovak flag displayed next to my name, not the German flag, and so I currently have only 1 tournament per month to play in, until WGT fixes this.
Also note what WGTalex wrote about this:
WGTalex:please note that we will need to disqualify any prize winners who did not correctly indicate their true country of residence.