Click in OK button inside an Alert (Selenium IDE)

Try Selenium 2.0b1. It has different core than the first version. It should support popup dialogs according to documentation:

Popup Dialogs

Starting with Selenium 2.0 beta 1, there is built in support for handling popup dialog boxes. After you’ve triggered and action that would open a popup, you can access the alert with the following:

Java

Alert alert = driver.switchTo().alert();

Ruby

driver.switch_to.alert

This will return the currently open alert object. With this object you can now accept, dismiss, read it’s contents or even type into a prompt. This interface works equally well on alerts, confirms, prompts. Refer to the JavaDocs for more information.

To click the "ok" button in an alert box:

driver.switchTo().alert().accept();

This is an answer from 2012, the question if from 2009, but people still look at it and there's only one correct (use WebDriver) and one almost useful (but not good enough) answer.

If you're using Selenium RC and can actually see an alert dialog, then it can't be done. Selenium should handle it for you. But, as stated in Selenium documentation:

Selenium tries to conceal those dialogs from you (by replacing window.alert, window.confirm and window.prompt) so they won’t stop the execution of your page. If you’re seeing an alert pop-up, it’s probably because it fired during the page load process, which is usually too early for us to protect the page.

It is a known limitation of Selenium RC (and, therefore, Selenium IDE, too) and one of the reasons why Selenium 2 (WebDriver) was developed. If you want to handle onload JS alerts, you need to use WebDriver alert handling.

That said, you can use Robot or selenium.keyPressNative() to fill in any text and press Enter and confirm the dialog blindly. It's not the cleanest way, but it could work. You won't be able to get the alert message, however.

Robot has all the useful keys mapped to constants, so that will be easy. With keyPressNative(), you want to use 10 as value for pressing Enter or 27 for Esc since it works with ASCII codes.