The normal way to install Maemo Mapper is to run Application Manager and see if it is listed in the installable programs listed there. If it's there, install it. If it isn't, you should go to someplace like http://www.gronmayer.com/it/ and install whatever repository has maemo mapper in it, which I think you can easily find from that site.

I didn't have to do that procedure, because I have already installed all the repositories from the /it/ site, so it was already in my installable programs when I looked for it.