Either go to Links Menu -> Import -> Link File Processor, or right click on MediLinkIt (taskbar notification area), and choose Link File Processor.
Add a ASCII v2 / v3 / v3 Audit4 PATIENTS.IN File Processor (ask your external software vendor what type they provide).
You will be asked if you want to update existing records and append new records. You may want to only update existing patients, but not add new ones in. Or you might want to only append new patients (i.e. Medilink may be more up to date but this external source has new patients). Or you may just want to have both on (which will update first, but if it cannot, then will append patients).
Set the Folder Path to be where the PATIENTS.IN file is being output to (again ask your external software vendor). Please note, avoid using the Medilink Data folder as this is where Medilink typically outputs it's own PATIENTS.IN file and that would cause problems.