Thank you very much Garvinhicking

You are right,i didn't install the event plugin of guestbook,the table:serendipity_guestbook created
But it's still do not work,the table is empty authough i reply the thread

Yes,the table serendipity_guestbook didn't exist,
following is the SQL:
SELECT timestamp, name, body FROM serendipity_guestbook ORDER BY timestamp DESC LIMIT 1

And the plugin installed,i could see it's info in the backoffice,such as :
Display the latest guestbook items in the sidebar
版本: 1.08[/img]

Which database type and version are you using?
I used mysql 5.0
And I found that there are several sites met this problem:
you could google :PLUGIN_GUESTSIDE_NOENTRIES

I found this plugin do not excute this function :
createTable() in serendipity_event_guestbook.php ,

it's should the PB why it could not work,
Is there anyone could tell me how to install a plugin in this blog system,thanks!

The key code in serendipity_plugin_guestbook.php

$sql = "SELECT ...

i downloaded this plugin,and installed in the blog,but it does not

work,show message:PLUGIN_GUESTSIDE_NOENTRIES

how to solove this PB?