The Hramina marina in Murter is located in the north Easter corner of the town at the following address Put Gradine 1, once in the city you can not miss it, but just in case here are the GPS co-ordinates 43.826639, 15.593330.
How to travel to marina Hramina.
Well that depends of from where you are arrive, assuming you are planning to fly to Croatia, here is the airports you should consider.
The are basically two suitable airport, one is the airport in Zadar which is about 85 km (1 hour travel time) from the marina other options is Split airport which is about 95 km from Marina Hramina (Travel time 1 hour 15 min). From both airport you can pre-oder a taxi or minibus transfer directly to the marina, offers from Zadar airport to Hramina you can find here, and prices from Split airport to marina Hramina here.
If you travelling during main summer time, please keep in mind that traffic jams are not uncommon, so make sure to have sufficient buffer in your itinerary.
Of course Croatia has more airport which you basically also could fly to but, but all alternative airport which is in this case is Dubrovnik, Rijeka, Zagreb and Pula are between 3 and half and 5 hours from the marina.
Bus or train to marina Hramina.
Travelling to Murter by train is not possible, the nearest train station is the one I Sibenik which is 35 away, so using train as transportation inside Croatia is not suitable when you have to get to Island Murter, but as mentioned it is possible to use train to get to Sibenik, from where you then will have to take a public bus or more likely a taxi to get to Hramina.
Travelling to Murter by bus is possible from Sibenik where there are a handful of daily connections during the summer season, also from Zagreb there are a few buses during July and August, from other towns you will have to travel via Sibenik. The bus stop in Murter is located within walking distance of the marina.