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Hi! I use version 3.0.1 for Win. When I try to use .png texture with transparent areas as metal or opaque material it doesn't work (no holes). Is there a solution for this problem?

That is true, the 3.x plugin uses Maxwell's new "Material Assistants" in some cases where previously it created materials from scratch, so some behaviors are different. In the case of the Metal Character, there may be something I can do to make this particular thing (opacity based on the use of a transparent PNG) work closer to how it used to, but not for the Opaque Character (which did not exist prior to 3.x, and therefore does not have different behavior, regardless). If you are using Maxwell Render Suite, there is always the choice of using a custom MXM file.

Can I use my v3 license to activate version 2.7. while metal material doesn't work with png?

I don't think so, but you can try using the 3.0.5 build I just uploaded (please see here and/or here), which adds support for it.