This blog is somewhat off topic however I have just spent an hour or so trying to reconnect Windows Live Writer to my SquareSpace V5 blog. It should and sometimes is straight-forward;
1) Add the URL to your blog
2) Add your username
3) Add your password
However you maybe asked to ‘Select Blog Type’. If this is the case, you need the following key information.
1) Select Metaweblog API
2) Enter http://www.squarespace.com/process/service/PostInterceptor as the Remote posting URL for your blog.