Module 10 - Selenium IDE: [error] locator not found

In Salesforce, I am trying to create a new Task. This specific problem is with opening the pop-up window for 'Subject'.

I am in 'New Task' window for Tom Hanks. In this window, I recorded clicking on icon next to Subject.
<tr><td>click</td><td>css=img.comboboxIconOn</td><td></td></tr>
But when I ran it, I got error "[error] locator not found: css=span.mruText"

Then I tried to 'Find' this element by clicking on Find button in IDE and got the same error.

So I changed Target in the above command to use XPath from Firepath: .//*[@id='ep']/div[2]/div[4]/table/tbody/tr[1]/td[2]/div/a/img
And ran this line again, but still got error: [error] locator not found: .//*[@id='ep']/div[2]/div[4]/table/tbody/tr[1]/td[2]/div/a/img
The Find command also gives same error.
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'Find' can find other elements like Status and Priority in the exact same New Task window.

I also tried with the downloaded code and have the same issue.
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The above is only an example - the same thing happened in a few other places and I was forced to skip these.

Can you pl. tell me how I can fix this error. I am stuck.