[Administration Note: This thread was moved under the "ProcessBook and Microsoft Office Development" forum.]
No objects in the ProcessBook automation model should be created with the new keyword other than the Application object. To create a new entry you'll need to call the Procbook.Entries.Add. ProcessBook will create a new Entry for you as the return value of the Add.
The Entry will be a text label initially. To change the entry to a link to a PDI call Entry.DefineAction. See the help topics on this method. One tricky part is the type parameter. See the help for the Entry.Type property for the correct values. If you're using ProcessBook 3.1 you can use the enumerated type pbEntryinkType in place of hardcoded numbers.
While not C#, here is a VBA macro example:
Dim book As ProcBook
Dim ent1 As Entry
Dim ent2 As Entry
Set book = Application.ProcBooks(1)
Set ent1 = book.Entries.Add("embedded")
ent1.DefineAction pbEntryLinkType.pbEntryPDIEmbeddedInLocalPIW, "", ""
Set ent2 = book.Entries.Add("link")
ent2.DefineAction pbEntryLinkType.pbEntryLinkedPDI, "C:\pbfiles\mydisplay.pdi", ""