Open Start Menu
-> Type in the search field "regedit" and press Enter
-> Browse to HKEY_CURRENT_USER\Software\Scirra\Construct2\html5
-> Normally it should exist, so just press right click on it, choose "Edit" and set it's value to 1.
-> If it doenst exist, rightclick in the empty window
---> New
---> DWORD Value
---> Set value to 1and name the value "devmode"