The file you posted is not a file you are supposed to open in Mathcad. Its part of a so called mathcad e-book. This *.hbk file is simple text file and represents sort of an index. It lists the names of all the mathcad-files which belongs to that handbook (in your case it consists of 13 files).
The *.hbk file should be located in the subdir "Handbook" of the installation directory of Mathcad (15 or below). The Mathcad files this handbook consists of, should be in a further subdir of handbook with the same name as the hbk-file, so e.g. they should be in "C:\program files\Mathcad\mathcad 15\Handbook\Ex2 FlatSlab\"
If the handbook is installed that way on your computer, from within Mathcad go to Help/E-Books and in the list you should find your handbook to browse. While they are normal MC sheets they are protected in some way to help preventing changing them. They are supposed for reference and you can copy expressions from there to your worksheet.