Further edited in light of revised information
It would seem that your professor is keen to have you continue the work of the previous developers. I'm not sure I agree with him on the ease of learning Struts2 vs SpringMVC, but I can see his point. He is clearly keen to stick with a chosen framework that is well understood within the faculty.
There is some backwards compatibility between Struts1.3 and Struts2 but it is limited and you may find that you are better served by just rewriting the original work in Struts2. Of course, this assumes that you are working within the web application, not just consuming it's output. If you are consuming it's output, then you don't care what it is using to manage views and page transitions internally, just so long as you can get the content.
In short, it appears that you're stuck with Struts so just make the best application you can with it.
Edited in light of comment
Given that you are a student, and the team leader of this project, then you must balance your lack of experience with the technology with the responsibilities that come with leading the team.
On the one hand, Struts2 is capable of solving your problem. It's a reasonable framework and will certainly introduce you to web application programming in Java in a manner that will be useful in the future.
On the other hand, I (and many other developers) favour SpringMVC because it makes it absolutely clear how the Model, View and Controller are interacting. It also introduces the overall Spring framework approach to your web application which promotes good coding practices throughout (dependency injection, externalisation of configuration etc).
As an aside, here is a brief overview of the key concepts in SpringMVC. Your application context contains the URI mappings to determine the entry points to your application (the Controllers) and the exit points (the Views). The Controllers call in to the services and business domain to get stuff done. The result of the work done by the services is turned into a ModelAndView object which is then used as the basis for the response. This response could be decorated by a JSP (in which case the contents of the ModelAndView is visible to the JSP) or could be directly marshalled as XML, or perhaps treated as a PDF or Image.
In answer to your question, I would go with informing your professor of your reservations with Struts2, but make sure you have an example of the same code written in SpringMVC to hand. Extol the virtues offered by Spring and do your best to win your case, both for yourself and the rest of the team who will be affected by your professors final choice.
If your professor is reasonable and you put your case well then you will learn how to deal with management for your future career - valuable experience indeed.
After chatting to numerous developers with experience with Struts2 I would counsel you away from it. There are better, more intuitive frameworks out there that just make the whole page navigation and security issues straightforward. I'm thinking SpringMVC and RESTEasy here.
However, you want to know about Struts2, so here is a handy tutorial that should work fine in NetBeans. There is a detailed alternative for SpringMVC which targets Eclipse should you want to go that way. Although Eclipse is a fabulous IDE, if you have more experience with NetBeans then stick with that until you're confident enough with your coding to make the move.
Make sure you understand what the Struts2 framework is doing for you over and above the underlying servlet API, and take time to understand HTTP and it's limitations.