If you want to point your AutoCAD to a linetype definition file other than the standard ACAD.LIN, resp. ACADISO.LIN, use the following steps.
AutoCAD loads by default the above mentioned .LIN file (imperial or metric "ISO", depending on the MEASUREMENT variable). So you may need to rename your custom file. Backup the original ACADISO.LIN file and rename your custom .LIN file to ACADISO.LIN (or ACAD.LIN if you use inches/feet).
Another option (e.g. for menu macros) is to use the commandline version of the command - -LINETYPE. With the command:
you can load all linetypes "MyLin*" from the file MyFile.lin. If you do not run this command from a macro and you want to type the .LIN filename, switch off the FILEDIA variable temporarily.