Assuming that you actually want to create a real test case, as opposed to a mock in a unit test (or similar), then this blog post has a code sample which creates one: http://www.ewaldhofman.nl/post/2009/12/11/TFS-SDK-2010-e28093-Part-5-e28093-Create-a-new-Test-Case-work-item.aspx.
Here's some hacked-together code that creates a new test case, based on the blog entry above, and then adds it to an existing test suite (called "ExampleSuite2", located in a test plan called "TfsVersioning Test Plan"):
var tfsUri = new Uri("http://localhost:8080/tfs/");
var tfsConfigServer = new TfsConfigurationServer(tfsUri, new UICredentialsProvider());
tfsConfigServer.EnsureAuthenticated();
var projCollectionNode = tfsConfigServer.CatalogNode.QueryChildren(new[] {CatalogResourceTypes.ProjectCollection}, false, CatalogQueryOptions.None).FirstOrDefault();
var collectionId = new Guid(projCollectionNode.Resource.Properties["InstanceId"]);
var projCollection = tfsConfigServer.GetTeamProjectCollection(collectionId);
ITestManagementService tms = projCollection.GetService<ITestManagementService>();
var project = tms.GetTeamProject("TfsVersioning");
var testCase = project.TestCases.Create();
testCase.Title = "Browse my blog";
var navigateToSiteStep = testCase.CreateTestStep();
navigateToSiteStep.Title = "Navigate to \"http://www.ewaldhofman.nl\"";
testCase.Actions.Add(navigateToSiteStep);
var clickOnFirstPostStep = testCase.CreateTestStep();
clickOnFirstPostStep.Title = "Click on the first post in the summary";
clickOnFirstPostStep.ExpectedResult = "The details of the post are visible";
testCase.Actions.Add(clickOnFirstPostStep);
testCase.Save();
//Add test case to an existing test suite
var plans = project.TestPlans.Query("SELECT * FROM TestPlan where PlanName = 'TfsVersioning Test Plan'");
var plan = plans.First();
var firstMatchingSuite = project.TestSuites.Query("SELECT * FROM TestSuite where Title = 'ExampleSuite2'").First();
((IStaticTestSuite)firstMatchingSuite).Entries.Add(testCase);
plan.Save();
After running this code the new test case appeared in MTM, associated with the target test suite.