Where can I ask questions about Catia ?
(for those who do not know, Catia is a very powerful 3D CAD/CAM/CAE drawing software)
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For the two questions you gave examples for, honestly, the best place to ask is on Google. For your first question:
How can I zoom in/out using just the mouse in Catia V5 ?
A search for "catia mouse zoom" yields a number of results including this tutorial video on rotate/pan/zoom with the mouse in CATIA V5, which appears to contain all the information you need and more.
For your second question:
What is a good training guide to learn Catia V5 ?
A search for "learn catia v5" yields more than a few nice looking results, some paid, some free. I'll leave it as an exercise to you to look through those results and find information that works for you.
Other dedicated CATIA forums can be found via a search for "catia forums".
For more serious issues, and presuming your copy of CATIA is legitimate, Dassault offers phone and email customer service.
This is not meant to be snarky, just a legitimate source of information. The internet has a wealth of information and, granted it sometimes has a low signal-to-noise ratio, usually information is easy to find; questions like the one you are asking may be met with resistance on SE sites because in general, SE is not a replacement for Google.
It is worth noting that I was able to find all of this quickly despite not being a graphic designer, not being particularly proficient with CAD, and never hearing of CATIA. So have no fear, the information is out there and is easy to find.
In general, it hurts the internet's SNR to ask easily searchable questions on SE sites -- the answers tend to consist of links back to the Google search result, and the search result will frequently have that answer at the top (due to SE having some weight in search results). The only thing accomplished there, then, is that one less slot is available on the front page of Google results.
So, research first, then ask! I like to refer people to this article about asking questions on the internet - it has some good general dos and don'ts that apply to all questions, not just technical. You may wish to read it (and pay special attention to step 1!).
I think you can ask on the Graphics Design SE site.
It depends on the question type.
Type 1: Highly engineering-related questions about the use of the modeling tools, production suitable drawings, physical material properties, etc. The appropriate place would be Mechanical Engineering SE or even CAD SE, which are unfortunately just proposals at area51 at the moment.
Type 2: Questions about lightning, texturing, post processing and also modelling may be appropriate on Graphic Design SE beta.