CSS #id ?

Check the form HTML again. The
tag wrapping the entire form has an id="" attribute, which is what Compact Forms looks for. The id attribute is optional so you might not find it on all forms...

<form action="/node?destination=node"  method="post" id="user-login-form">
...
...
...
</form>

The above example is taken from the user login block. In this case I enter 'user-login-form' on the settings page (done by default).

Form ID:s:
user-login-form

What if I want the behaviour on the user register form as well? That form looks like this:

<form action="/user/register"  method="post" id="user-register">
...
...
...
</form>

Hence, you add 'user-register' on the settings page:

Form ID:s:
user-login-form
user-register

Hope that clarifies the idea. I'm thinking of adding checkboxes for common Drupal forms in the next release, to make the process even easier.