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Hi,

There is no Design Center in AutoCAD for Mac. But you can manage block libraries with Content Palette:

http://lynn.blogs.com/lynn_allens_blog/2011/01/autocad-for-mac-content-palette.html

http://lynn.blogs.com/lynn_allens_blog/2011/01/managing-content-libraries-on-autocad-for-mac.html

As about other libraries (linetypes, text and dimension styles, etc) - the only one way is to include them in custom Template which you can use for creating new drawings.

Maxim