I'm guessing you're talking about the java.util.Deque interface. That would have been useful to mention, I had to hunt around the javadocs a bit to find which collection class/interface it might be :-)
If you have an object implementing Deque, then both methods will do the same. The difference is that using add will allow you to use an object that implements Queue instead, since add is part of both interfaces, whereas addLast is only part of Dequeue.
Why would you want to do this? Well, there may come a day when you decide that you didn't need to use a Deque after all, and would like to switch to Queue instead (maybe in order to use other collection classes that implement Queue, but not Deque). Had you used addLast, then you would need to refactor your code to use add instead; if you had used add from the beginning, then there is no need to change anything.