The first thing you need to do is to point Flock to your weblog.

Flock will encounter troubles trying to automatically detect your blog's settings. Hit "Cancel" to manually configure your weblog.

Choose the "Movable Type API" (do not choose the one labelled "_movable type api" if it exists).

Your final configuration should look something like below. For help trying to find your blog's ID, instructions are at How do I use other blogging clients.

Finally, enter your credentials for your weblog.

And, you're done!