usdm is a very unpopular coin, transaction volume is very low with such conditions of course exchangers that can support this coin are very low.
I see on coinmarketcap there are only 3 exchangers that support it, of course the official exchanger site from usdm itself, the other 2 are digifinex and l bank.
https://coinmarketcap.com/currencies/usd-mars/#Markets