Modest provincial airfields in Papua New Guinea's Southern Highlands do not carry lounges, and Mendi Airport (MDU) is no exception. If you were hoping for a business lounge at this field, the honest answer is that none exists.
Air travel through Mendi follows the wider Highlands pattern: traffic connects mainly to Mount Hagen's Kagamuga Airport, the region's larger hub in Western Highlands Province, and onward to Port Moresby's Jacksons International Airport for national and international connections. Traffic through this chain is not steady year-round; it noticeably spikes around major regional events, most visibly the Mount Hagen Cultural Show, when significantly more travelers move through the Highlands airports than on an ordinary week. If your trip lines up with an event like that, build in extra time regardless of lounge access.
It is worth checking your government's current travel guidance before a Highlands itinerary as well, since some governments note added caution for travel in parts of Papua New Guinea, including highlands provinces, beyond the general advice given for the country overall. That guidance is worth more to your planning than any lounge question at a field like Mendi.
For any real lounge access on a Highlands trip, Mount Hagen is the realistic pivot point, though keep expectations modest even there: this is a regional PNG hub rather than a major international airport, and any lounge space is more likely tied to a specific airline's status passengers than open to a broad card network. Priority Pass coverage should not be assumed at any Highlands airport without checking directly. At Mendi itself, plan on basic terminal seating, a schedule shaped by the day's connecting flights, and any comfort-focused planning saved for Port Moresby or wherever your itinerary next reaches a larger airport. Highlands connections answer to weather and aircraft rotations well before they answer to a printed timetable, so build in a margin either way.