FindBugs not showing the bugs found

I've used FindBugs before and I didn't have problems, however this time on Eclipse Helios on Ubuntu Natty Narval bugs are found but not shown.

Here's what I tried:

• Right Click on the project > Find Bugs > Find Bugs
• Looking at the progress I saw 3 bugs were found
• In the Problems view they are not listed

I tried to go to its own perspective with: Open Perspective > Other > FindBugs but Bug Explorer is empty.

Any ideas to view those bugs?

UPDATE: In Eclipse error log I see:

!ENTRY edu.umd.cs.findbugs.plugin.eclipse 2 2 2011-06-18 21:13:24.968
!MESSAGE The following classes needed for FindBugs analysis on project **
     were missing:
!SUBENTRY 1 edu.umd.cs.findbugs.plugin.eclipse 2 0 2011-06-18 21:13:24.969
!MESSAGE org.apache.commons.exec.CommandLine
!SUBENTRY 1 edu.umd.cs.findbugs.plugin.eclipse 2 0 2011-06-18 21:13:24.969
!MESSAGE org.apache.commons.exec.DefaultExecutor
!SUBENTRY 1 edu.umd.cs.findbugs.plugin.eclipse 2 0 2011-06-18 21:13:24.969
!MESSAGE org.apache.commons.exec.ExecuteException

UPDATE 2: I don't know how to fix Eclipse, but I found a workaround. I added the findbugs report to maven's pom.xml

<reporting>
    <plugins>
        [...]
        <plugin>
            <groupId>org.codehaus.mojo</groupId>
            <artifactId>findbugs-maven-plugin</artifactId>
            <version>2.3.3-SNAPSHOT</version>
            <configuration>
                <xmlOutput>true</xmlOutput>
                <!-- Optional derectory to put findbugs xdoc xml report -->
                <xmlOutputDirectory>target/site</xmlOutputDirectory>
            </configuration>
        </plugin>
    </plugins>
</reporting>

The following configuration generates an HTML report in target/site/findbugs.html