How to access unmanaged dlls in Unity

I'm trying to access data from a Garmin heart rate monitor in Unity via ANT USB stick. I can successfully receive data from it using a Visual Studio C# console application, but I cannot connect to the USB stick when I use the application as a script in Unity. I believe this is a result of Unity not correctly referencing the C/C++ dlls. I can successfully reference the C# dll (ANT_NET.dll) by placing it in the assets folder, but the other (unmanaged) dlls seem to be missing. This is problematic because it seems that ANT_NET.dll is dependent on them. In my console application, I can just include them in the working directory and they function without issue. What do I need to do to access these dlls in Unity? I am currently using a trial of Unity Pro if plugins are required.

Any suggestions are greatly appreciated. Thanks.