Depending on which direction you are trying to go with it... but you need to start with a UserInfo object (DotNetNuke.Entities.Users namespace):

UserInfo someInfo = new UserInfo(); // set up a new user, for example
user.Profile.InitialiseProfile(PortalId); // initialize profile
// ... set some properties...
user.Profile.SetProfileProperty("somePropertyName", "somePropertyValue");

Or to retrieve properties, again, you need to have a UserInfo... but then instead of SetProfileProperty, you can GetPropertyValue("somePropertyName") -- user.Profile.GetPropertyValue("someValue");  it'll return it as a string using that so you will need to cast/convert to the appropriate type.