Error running on Win10

Hi,

I have an issue running MPI.jl on windows. In the Julia REPL, first I typed
```
julia> using Pkg

julia> Pkg.add("MPI")
Updating registry at `C:\Users\henry\.julia\registries\General`
Updating git-repo `https://github.com/JuliaRegistries/General.git`
Resolving package versions...
Updating `C:\Users\henry\.julia\environments\v1.4\Project.toml`
[no changes]
Updating `C:\Users\henry\.julia\environments\v1.4\Manifest.toml`
[no changes]

julia> Pkg.status()
Status `C:\Users\henry\.julia\environments\v1.4\Project.toml`
[da04e1cc] MPI v0.15.0
```

Then I created the same demo script from [here](https://juliaparallel.github.io/MPI.jl/latest/usage/):
```
# examples/01-hello.jl
using MPI
MPI.Init()

comm = MPI.COMM_WORLD
println("Hello world, I am $(MPI.Comm_rank(comm)) of $(MPI.Comm_size(comm))")
MPI.Barrier(comm)
```
During my first attempt in `cmd`,
```
mpiexec -3 julia test.jl
```
it returned similar as being reported in this [issue](https://stackoverflow.com/questions/46209625/mpiexec-credentials-for-user-rejected-connecting-host)
```
User credentials needed to launch processes: account (domain\user) [DESKTOP-3CFSBJ8\Hazem]:
```

I followed the instructions there
```
mpiexec -register
```
and added a user and passwd. Then, for the second attempt,
```
Microsoft Windows [Version 10.0.19041.388]
(c) 2020 Microsoft Corporation. All rights reserved.

D:\Test>mpiexec -n 3 julia test.jl
ERROR: ERROR: ERROR: LoadError: LoadError: LoadError: ArgumentError: Package MPI not found in current path:
- Run `import Pkg; Pkg.add("MPI")` to install the MPI package.

Stacktrace:ArgumentError: Package MPI not found in current path:
- Run `import Pkg; Pkg.add("MPI")` to install the MPI package.

Stacktrace:ArgumentError: Package MPI not found in current path:
- Run `import Pkg; Pkg.add("MPI")` to install the MPI package.

Stacktrace:
[1]
[1] require
[1] requirerequire(::(::(::Module, ::Symbol) at .\loading.jl:892
[2] include(::Module, ::String) at .\Base.jl:377
[3] exec_options(::Base.JLOptions) at .\client.jl:288
[4] _start() at .\client.jl:484
in expression starting at D:\Test\test.jl:1
Module, ::Symbol) at .\loading.jl:892
[2] include(::Module, ::String) at .\Base.jl:377
[3] exec_options(::Base.JLOptions) at .\client.jl:288
[4] _start() at .\client.jl:484
in expression starting at D:\Test\test.jl:1
Module, ::Symbol) at .\loading.jl:892
[2] include(::Module, ::String) at .\Base.jl:377
[3] exec_options(::Base.JLOptions) at .\client.jl:288
[4] _start() at .\client.jl:484
in expression starting at D:\Test\test.jl:1
```

What is the exact error here?