How do I get the equivalent of Set-DnsClientServerAddress in Windows Server 2008 R2?

It's not possible to do this via PowerShell (the docs aren't explicit about this, but you can see Server 2008 is not listed at the top of the page), but you can use netsh:

netsh interface ip add dnsserver "Local Area Connection" 10.0.0.1

Since you asked for PowerShell, you can use WMI and set it via the Win32_NetworkAdapterConfiguration provider:

Get-WMIObject win32_NetworkAdapterConfiguration

This will list the adapters you have, then you can select a specific one:

$adapter = Get-WMIObject win32_NetworkAdapterConfiguration -Filter "Index=X"
# or use PowerShell filtering
$adapter = Get-WMIObject win32_NetworkAdapterConfiguration | Where-Object { $_.Index -eq X }

Then you use the SetDNSServerSearchOrder() method to to set the address(es):

$adapter.SetDNSServerSearchOrder("8.8.8.8")
# or a list
$dns_servers = "8.8.8.8", "8.8.4.4"
$adapter.SetDNSServerSearchOrder($dns_servers)

Bonus one-liner:

(gwmi win32_networkadapterconfiguration -Filter "index=x").SetDNSServerSearchOrder(@("8.8.8.8","8.8.4.4"))