Normally, you just need to select the desired users within the "Editors" tab in the Edit Post form.

How have you modified capabilities via Role Manager?  In most cases, the modification is not necessary and you will have more success by restoring the default capabilities, then using Role Scoper to assign Post-specific or category-specific roles.

If your role definition (via Role Manager) must contain some custom capability, you can make Role Scoper use that (with some limitations) by navigation to Roles > Options and setting "Role Type" to "WP"