Sorry, my first message was meant to be addresed to the list.
Could you try with the latest svn version of the extension script and give us feedback?
http://inkscape.svn.sourceforge.net/viewvc/inkscape/inkscape/trunk/share/ext...
It is better, I have now a window message :) :
dvips: ! DVI file can't be opened.
[…]
Yes, it's better!
Now I guess you got this message because the formula input you gave to the extension was either not valid LateX, or it became invalid because of a shell expansion which occured when the script was called by the extensions manager (this is a bug).
Can you try with a simple "( \cos(x) )" LaTeX input?
To know how your imput will be expanded you can use the following shell command: $ echo -E "$$\cos(x)$$" 8012\cos(x)8012
For example '$$' is expanded to the pid of the processus. As a workarround, try not to use '$' or '$$' but '(' and '[' instead to enter math mode.
Stéphane.
PS: I'll also try to improve error reports when the given input is not valid LaTeX.