how to access the config file from workflow assembly

my workflow assembly now works fine. But I put hard code for url and
userid, such as

_service = new CrmService();
_service.Url = "http://crm/mscrmservices/2006/crmservice.asmx";
_service.Credentials = System.Net.CredentialCache.DefaultCredentials;
_service.CallerIdValue = new CallerId();
_service.CallerIdValue.CallerGuid = new
Guid("3a4bbf99-f0e7-4b91-0000-a116a75ea665");

I tried to use app.config and web.config to store the data, such as

<appSettings>
<add key="DBA_WebserviceUrl"
value="http://crm/mscrmservices/2006/crmservice.asmx" />
<add key="DBA_WorkflowUserId"
value="3a4bbf99-f0e7-4b91-0000-a116a75ea665" />
</appSettings>

But I try to call use the fllowing code in workflow assembly

the _service.Url =
ConfigurationSettings.AppSettings["DBA_WebserviceUrl"];


But it does not work.

Thanks