Nicolas,
I think you need to have input (and result) types tupled (that is, only one input and one output parameter).
The first example works because you have a 1-tuple in input and a 1-tuple in output.

You can change

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float -> float -> float 

to

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(float * float) -> float 

and possibly (not sure here)

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(float -> float) -> float -> float 

to

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((float -> float) * float) -> float 

.

It would be better to expose e.g. System.Func delegates from an F# library if you expect the F# library to be called from C#.

That said, a type like "int -> int -> int" is just a series of one argument functions that may return other functions, so e.g. (+) is like

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    var g = Microsoft.FSharp.Core.FuncConvert.ToFSharpFunc((int y) =>
    {
        var f = Microsoft.FSharp.Core.FuncConvert.ToFSharpFunc((int x) => x + y);
        return f;
    });
    g.Invoke(4).Invoke(3);