The Axure handwriting font can be downloaded here. Install it as you would any other font on your computer through the control panel.
http://axure.cachefly.net/AxureHandwriting.zip
Keep in mind that although you may see the font, the user on the other end will likely not have this font, and therefor it will render as a default font. The solution for this problem is so select “Advanced > Render text panels as images” in the Generate > Prototype dialog.