how to get MAC address of remote computer

nmap will return the MAC address as well as just about anything else you'd like to know.

If you have admin access to the machine, powershell & wmi are both very useful in getting remote diagnostics. They both have extensive documentation at technet.microsoft.com

edit: this assumes a windows machine, which from the looks of it, this might not be.

MAC addresses are Ethernet things, not Internet things. A computer need not even have a MAC address. The only way to get the MAC address is to get some computer on the same LAN as that computer to tell it to you. And you'd have no way to know it was giving you the correct information.

If the two of you are in the same Ethernet LAN, you can just ping the computer and then look in your ARP table. Otherwise, you would have to ask a computer in the same Etherent/Wifi LAN.

You can get it from WMI, and any language that can read WMI will be able to access it. VBScript, JScript, Perl, Python, and Powershell can all be used to get to it.

Since you asked specifically Powershell, here's an example from http://www.neolisk.com/techblog/powershell-getmacaddressofanyremoteip:

param ( $Computer , $Credential )
#to make it work without parameters
if($Computer -eq $null) { $Computer = $env:COMPUTERNAME }
#program logic
$hostIp = [System.Net.Dns]::GetHostByName($Computer).AddressList[0].IpAddressToString
if($Credential) {
    $Credential = Get-Credential $Credential
    $wmi = gwmi -Class Win32_NetworkAdapterConfiguration -Credential $Credential -ComputerName $Computer
} else {
    $wmi = gwmi -Class Win32_NetworkAdapterConfiguration -ComputerName $Computer 
}
return ($wmi | where { $_.IpAddress -eq $hostIp }).MACAddress