Two methods:
In Setup - Settings open the Symbol Libraries node and add a new library using the Add new button and browse to the location of the project file (if the dialog hasn't already filled in the location). This should set the path and Alias. Change the path to a . (dot/full stop character) but leave the alias as is. Make the library writeable and then press OK. Move the library to the top of the list using the arrows and then press OK.
Alternatively edit the project file (name.prj) and add the ". 'library alias'" to the IndependentLibraries section of the project file, use the same syntax for the alias as your other libraries. To make it writeable the format should be:
LIST IndependentLibraries
VALUE "DIR [W] . (Your alias here)"