If you are using Google Chrome, right click on the button, and select Inspect in the popup. The html will open in a developer tools frame. Right click on the element in the developer tools frame, hover over copy, select copy xpath.

Here is the XPath to the form on the URL.

//*[@id="main-frame"]/div[1]/div[2]/div/div[1]/div[1]/form/regform-steps/ng-transclude/regform-step[1]/ng-form/ng-transclude/regform-button/button

    WebDriver driver = new ChromeDriver();
    driver.get("https://uk.match.com/unlogged/landing/2016/06/02/hpv-belowthefold-3steps-geo-psc-bowling?klid=6740");
    //fill in fields
    WebElement element = driver.findElement(By.xpath("//*[@id=\"main-frame\"]/div[1]/div[2]/div/div[1]/div[1]/form/regform-steps/ng-transclude/regform-step[1]/ng-form/ng-transclude/regform-button/button"));
    element.click();

You can use document.evaluate:

Evaluates an XPath expression string and returns a result of the specified type if possible.

It is w3-standardized and whole documented: https://developer.mozilla.org/en-US/docs/Web/API/Document.evaluate

function getElementByXPath(path) {
  return document.evaluate(path, document, null, XPathResult.FIRST_ORDERED_NODE_TYPE, null).singleNodeValue;
}

console.log( getElementByXPath("//html[1]/body[1]/div[1]") );

<div>foo</div>

https://gist.github.com/yckart/6351935

There's also a great introduction on mozilla developer network: https://developer.mozilla.org/en-US/docs/Introduction_to_using_XPath_in_JavaScript#document.evaluate

Alternative version, using XPathEvaluator:

function getElementByXPath(path) {
  return new XPathEvaluator()
    .createExpression(path)
    .evaluate(document, XPathResult.FIRST_ORDERED_NODE_TYPE)
    .singleNodeValue
}

console.log( getElementByXPath("//html[1]/body[1]/div[1]") );

<div>foo/bar</div>

You don't have your XPath syntax right. You need quotes round the text attribute values you're matching against. Try:

d.findElement(By.xpath("//select[@id='category']/option[@id='cat2']")).click();