I think I'm sorted. I was already using the external AIR library. So I did the opposite of Vilter's advice and set LIBRARY_IS_AIR to 0. I can now see my library, though I haven't used it yet .... (crosses fingers).

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Hardware: AMD Ryzen 9 3900X/MSI MAG570 Tomahawk X570/Zotac Geforce GTX 1650 Super 4GB/32GB OLOy RAM

Software: Windows 10 Professional/Poser Pro 11/Photoshop/Postworkshop 3