Sorin, these documents can be found at the webMethods Product Downloads page. For example, when I go to the Product Download page it shows me “DataDirect: Sequelink Server” and if I select this link I see links for “DataDirect: SequeLink Server - v5.1” (and v5.2 and v5.3). Selecting one of these links will give me all the documentation for the version of Sequelink.
If you do not see “DataDirect: Sequelink Server” on your download page, send an email to the webMethods Keymaster (email@example.com). They should be able to add it to the list on your download page.