Go to http://textpattern.com/download-rpc and download the gzip or zip of the XML-RPC code.
2.
Uncompress the package on your computer.
3.
Connect to your server with a FTP client.
4.
Open the textpattern folder, and then the libs folder. Find the file called txplib_wrapper.php in the folder you just uncompressed.
Upload that file to the directory at /textpattern/libs/ on your server.
5.
Upload the rpc directory from the uncompressed folder to the same level as your Textpattern directory.
6.
If you have Textpattern installed at the document root of your web server it will be accessible via http://mydomain.com/rpc. If you have Textpattern installed in a folder called blog, it will be accessible via http://mydomain.com/blog/rpc.
Now you can configure a client like MarsEdit to edit your website.