Have you added the SlickUpload section definitions inside the <configSections> tag at the top of your web.config? You should have something like this:
<configuration>
<configSections>
<!-- ...
Other config sections here
... -->
<sectionGroup name="slickUpload" type="Krystalware.SlickUpload.Configuration.NameValueConfigurationSectionHandler, Krystalware.SlickUpload">
<section name="uploadParser" type="Krystalware.SlickUpload.Configuration.NameValueConfigurationSectionHandler, Krystalware.SlickUpload"/>
<section name="uploadStreamProvider" type="Krystalware.SlickUpload.Configuration.NameValueConfigurationSectionHandler, Krystalware.SlickUpload"/>
<section name="statusManager" type="Krystalware.SlickUpload.Configuration.StatusManagerConfigurationSectionHandler, Krystalware.SlickUpload"/>
</sectionGroup>
</configSections>