To use Import Content, you will need to create an appropriately formatted XML file, not a CSV file.
I would create a module with a single announcement, and use the Export Content function to create an XML file. Then, you can use this to create an XML file that you can import for other announcements. If you do, you will get a file that looks like this:
<?xml version="1.0" encoding="utf-8" ?>
<content type="DNNAnnouncements" version="04.00.01">
<Announcements>
<Announcement>
<ItemId>3</ItemId>
<ModuleId>1884</ModuleId>
<Title>Announcement Title</Title>
<Url />
<ViewOrder>-1</ViewOrder>
<Description><p>Announcement Text</p></Description>
<ImageSource />
<TrackClicks>False</TrackClicks>
<NewWindow>False</NewWindow>
<PublishDate>7/4/2013 12:00:00 AM</PublishDate>
<ExpireDate>1/1/0001 12:00:00 AM</ExpireDate>
</Announcement>
</Announcements>
</content>
You can include multiple Announcement nodes in the file to import multiple announcements at once.