driver.findElement(By.xpath("//input[@accesskey='9']"));

td style="width: 5%;">
<input type="checkbox" data-bind="attr: { accesskey: Id }" accesskey="6">
</td>
<td style="width: 5%;">
<input type="checkbox" data-bind="attr: { accesskey: Id }" accesskey="7">
</td>

public void portfolioRenewalSearch(String portfolioId) throws Exception {

        try {
            driver.findElement(By.xpath("//input[@accesskey= portfolioId]"))
                    .click();
        } catch (AssertionError Ae) {
            Ae.printStackTrace();
        }
    }

public void portfolioRenewalSearch(String portfolioId) throws Exception {
    try {
        driver.findElement(By.xpath("//input[@accesskey="+portfolioId+"]"))
                .click();
    } catch (AssertionError Ae) {
        Ae.printStackTrace();
    }
}

Here is the solution for index variable

Let's say, you have found 5 elements with same locator and you would like to perform action on each element by providing index number (Here, variable is used for index as "i")

for(int i=1; i<=5; i++)
{
    string xPathWithVariable = "(//div[@class='className'])" + "[" + i + "]";
    driver.FindElement(By.XPath(xPathWithVariable)).Click();
}

It takes XPath as below
(//div[@class='className'])[1]
(//div[@class='className'])[2]
(//div[@class='className'])[3]
(//div[@class='className'])[4]
(//div[@class='className'])[5]