It's fairly easy, theoretically speaking.
IDM seems to be a COM library,
all you need to do is throw the TLB file into tlbimp and it will generate C# lib.
How to: Create COM Wrappers:
http://msdn.microsoft.com/en-us/library/ms404285(v=vs.110).aspx
You can use tlbimp.exe for this. In my computer there is this file:
C:\Program Files (x86)\Internet Download Manager\idmantypeinfo.tlb
You now have to run tlbimp command to generate the specific C# library:
C:\Program Files (x86)\Internet Download Manager>tlbimp idmantypeinfo.tlb
Microsoft (R) .NET Framework Type Library to Assembly Converter 3.5.30729.1
Copyright (C) Microsoft Corporation. All rights reserved.
Type library imported to IDManLib.dll
C:\Program Files (x86)\Internet Download Manager>
You now have generated IDManLib.dll that contains the necessary stuff.