The svg files have been created in either Adobe Illustrator CS6 or Adobe Illustrator CC. Attempting to open these files in an earlier version of Adobe Illustrator results in an error message -
'The operation cannot complete because of an unknown error'.
This problem can be resolved in one of two ways.
• update to either Adobe Illustrator CS6 or Adobe Illustrator CC
• download the free open-source application Inkscape.
Inkscape is a vector graphics editor similar to Adobe Illustrator, Corel Draw, Freehand or Xara X. It is cross-platform, so runs on Windows or Mac OS X. The svg files can be opened in Inkscape and edited, or saved as eps files which can be opened in Adobe Illustrator CS5. Unfortunately Inkscape collapses all the Illustrator layers into one layer, but editing is still possible.
See Tavmjong Bah's Inkscape: Guide to a Vector Drawing Program - available as a print or free digital edition.