While the CDC and other health departments had been monitoring the situation and running tests, the first report of the new virus in the English-language press occurred on April 21, 2009 (CDC 2009 H1N1 Pandemic Timeline). By April 29, the WHO raised the alert level for the virus to level five, which means that human-to-human spread is occurring in at least two countries, and a pandemic is imminent (WHO Pandemic Alert).
At this time, Carleton either created or updated its "Influenza Updates" page. Live by May 1, 2009, the site contained links to more information about H1N1 from authoritative health organizations, assurance that Carleton was monitoring the situation, and updates on the impact on travel (at that time, there were none).
The adminstration also took more active steps to inform the campus community about the situation. Carleton students received an email from the dean of students on April 28 about the virus and further communication from President Robert Oden on May 1, as shown on the "Campus News" section of the Influenza Updates page.
(Unfortunately, I have been unable to find any archived versions of those emails themselves)
On May 1, the first discussion of H1N1 appeared in the Carletonian, in the form of an editorial. The article briefly discusses the still-emerging virus, and provides advice to prevent its spread: washing hands, covering coughs and sneezing, and staying home when symptomatic.