New-SPWebApplication application pool account is not found

You must create a managed account first for your win2k8r2sptest\administrator. Here is the sequence of steps

$cred = get-credential 'win2k8r2sptest\administrator' // enter the password here
$adminMA = New-SPManagedAccount -Credential $cred
New-SPWebApplication -Name 'Test Webapp' -ApplicationPool "DefaultAppPool" -ApplicationPoolAccount $adminMA

I've tested this on vanilla SPS2010 SP1. I think the process is identical to Foundation as well.