First, I must say that using an XPath like that is strongly discouraged. This is because it is very fragile. Slight changes to the DOM will break your test.
The suggestion to use link text is good. Unless you need to test localized versions of this web app, then you should look for alternatives.
You did not mention what you had tried. So I'm going to guess what the issue might be.
If you want to start from the root of the document (html), the xpath you posted is fine.
But in order to use it, you need to specify the locator type explicitly.
Click Element xpath=html/body/div[1]/div[3]/div[1]/div/div[3]/div[1]/div[1]/div/div/div[2]/div[6]/div/div[2]/div/div/div/ul/em/li[1]/a/strong
Click element is used here because you are trying to click on a strong element. Click Link filters elements and will only work on a (anchor) elements.
Perhaps that is your issue.
This will probably work:
Click Link xpath=html/body/div[1]/div[3]/div[1]/div/div[3]/div[1]/div[1]/div/div/div[2]/div[6]/div/div[2]/div/div/div/ul/em/li[1]/a
The reason you must specify xpath= is because your xpath does not start with //.
Here's some other XPaths that would also need the locator prefix:
(//a)[5]
id('someTable')/th/td[2]