No problem - use this map in conjunction with the directions that follow!
http://www.multimap.com/map/browse.cgi?client=public&X=580000.225224578&Y=295000.322771136&width=700&height=400&gride=582027.225224578&gridn=291126.322771136&srec=0&coordsys=gb&db=freegaz&addr1=&addr2=&addr3=&pc=&advanced=&local=&localinfosel=&kw=&inmap=&table=&ovtype=&keepicon=true&zm=0&scale=50000&multimap.x=542&multimap.y=295
As you drive down the road to the arboretum having turned off the A1065 in Mundford, the main carpark is on the left, but there is also a carpark for orange badge holders (or similar) opposite it on the right. Both are shown arrowed on the map.
From here, it's a fairly short and easy walk straight down the edge of the arboretum, heading south to the bridge at the east end of the lake (lake not shown on the map, but the bridge is under the A of Arboretum). Keep going over the bridge and the paddocks are on your right as you walk towards the church (though no need to go much more than 100 yards past the bridge usually).
Check the tops of the hornbeam trees for Hawfinches, and listen out for the electric 'zit' call as they fly in - with a bit of patience and reasonable weather, you should get good views. The numbers have been excellent this year - I heard someone say they'd seen over 30 there today!