Are you refering to the openworklist function from the SourceCode.Workflow.Client API?  If so, this is for certain users.

For a management level view, please use the SourceCode.Workflow.Management API:

http://help.k2.com/onlinehelp/k2five/DevRef/5.0/default.htm#Runtime/WF-Manage/GlobalWorklist.html

i've tried what you suggest, but i got error

Authentication Failed : -2146892976

Where/How are you running this code?

How is your connection string configured?

If you are using this:

SourceCode.Hosting.Client.BaseAPI.SCConnectionStringBuilder connectionString = new SCConnectionStringBuilder();
connectionString.Authenticate = true;
connectionString.Host = "localhost";
connectionString.Integrated = true;
connectionString.IsPrimaryLogin = true;
connectionString.Port = 5555;

This will have to be ran on the K2server as the account  with K2 Server Admin rights; since it is "localhost" and integrated=true; if this is console/.NET application app that you are testing.

If this code is running as a web application, are you using an application pool account?  The site authentication will likely also play a role in the authentication.

https://help.k2.com/onlinehelp/k2blackpearl/devref/4.7/default.htm#ConnectionStringSamples.htm