Protel 99se uses .DDB databases for entire project. Of course, at import/export functions, several filetypes are supported (.lib, .sch) from/to several (orcad, pcad, etc.). If your version does not know .DDB files, surely it is not 99/99SE. Look at help, version, see what's there. Most probably you have an older version. After 99SE, comes DXP and 2004, which is the last version.