You can't, and shouldn't, directly look in the database with Selenium IDE. You can, and should, indirectly test that the transaction happened.
When you use Selenium, or any other browser controlling or simulating testing tool, you're testing your app from the user's point of view (writing a 'functional' or 'acceptance' test). Users don't know what's in the database -- they just know what they see on the screen. So if doing something in your web app changes something on some page of your web app that a user can see, go there in Selenium and test that.
This is the right thing to do even if your testing tool does have a way to look in the database.
If your app writes some kind of information to the database that is never shown to any user, such as auditing information, the only way to test that with Selenium IDE is to add a page (appropriately authenticated!) to your web app so that Selenium IDE can see it. If you aren't able to do that, you need a testing tool that can look in the database.
RSpec and Cucumber, with the Capybara Ruby gem, are examples of testing tools that can both test an application through its user interface and also look in (and even change) the database.