pt-table-checksum - Diffs cannot be detected because no slaves were found (1)
According to the Documentation on --recursion-method


  Preferred recursion method for discovering replicas. Possible methods
  are:

METHOD       USES
===========  ==================
processlist  SHOW PROCESSLIST
hosts        SHOW SLAVE HOSTS
dsn=DSN      DSNs from a table

  
  The processlist method is the default, because SHOW SLAVE HOSTS is not
  reliable. However, the hosts method can work better if the server uses
  a non-standard port (not 3306). The tool usually does the right thing
  and finds all replicas, but you may give a preferred method and it
  will be used first.
  
  The hosts method requires replicas to be configured with report_host,
  report_port, etc.
  
  The dsn method is special: it specifies a table from which other DSN
  strings are read. The specified DSN must specify a D and t, or a
  database-qualified t. The DSN table should have the following
  structure:

CREATE TABLE `dsns`
(
   `id` int(11) NOT NULL AUTO_INCREMENT,  
   `parent_id` int(11) DEFAULT NULL,
   `dsn` varchar(255) NOT NULL,  
   PRIMARY KEY (`id`)
);

  
  To make the tool monitor only the hosts 10.10.1.16 and 10.10.1.17 for
  replication lag and checksum differences, insert the values
  h=10.10.1.16 and h=10.10.1.17 into the table. Currently, the DSNs are
  ordered by id, but id and parent_id are otherwise ignored.


You could do this setup for DSN if you want, but you do not have to go that far yet. Why?

The default --recursion-method is the processlist. This simply means that you did not run START SLAVE; on the port 3307 instance. You cannot use --recursion-method processlist on a Master where all the Slaves are stopped. When you run START SLAVE; on port 3307, a DB Connection should appear in the processlist of the Master (3306 instance) from the Slave (3307 instance). In light of this, you should just run START SLAVE; on port 3307 and then run the pt-table-checksum.

Give it a Try !!!