Reading the TechNet docs on Add, edit, remove and display keywords by using Windows PowerShell (FAST Search Server 2010 for SharePoint) was a little confusing. Specifically the important note about making sure that you have a Search Settings Group. When you create or find a Search Settings Group you do so using a –Name parameter. The is nothing there to suggest what this Name parameter corresponds to.

However with a PowerShell hacking I found that this name is actually the GUID of the Site Collection for which you want to add keywords and best bets. Now armed with this info I was able to craft up this PowerShell script to do my bidding.

# SharePoint cmdlets
Add-PSSnapin Microsoft.SharePoint.PowerShell
if([Reflection.Assembly]::LoadWithPartialName("Microsoft.SharePoint")-eq$null){throw "Unable to load Microsoft.SharePoint.dll";}

Add-PSSnapin Microsoft.FASTSearch.PowerShell
if([Reflection.Assembly]::LoadWithPartialName("Microsoft.FASTSearch.PowerShell")-eq$null){throw "Unable to load Microsoft.FASTSearch.dll";}

Start-SPAssignment -Global            # This cmdlet takes care of the disposable objects to prevent memory leak.

#get the Search Settings Group for the Site Collection
$site = Get-SPSite -Identity "http://northwindtraders"
$ssg = Get-FASTSearchSearchSettingGroup -Name $site.ID

if($ssg -eq $null)
{
	#create the ssg if it doesn't exist
	$ssg = New-FASTSearchSearchSettingGroup -Name $site.ID
}

$ssg.Keywords.AddKeyword("snowmobile")
$keyword = $ssg.Keywords.GetKeyword("snowmobile")
$uri = New-Object -TypeName System.Uri -ArgumentList "http://northwindtraders/products/snowmobiles/pages/snm881.aspx" 
$keyword.AddBestBet("SNM881", "A large sled with plenty of storage", $uri)
$uri = New-Object -TypeName System.Uri -ArgumentList "http://northwindtraders/products/snowmobiles/pages/snm313.aspx" 
$keyword.AddBestBet("SNM313", "Our most powerful sled. Extreme Power!", $uri)

Stop-SPAssignment -Global

Remove-PsSnapin Microsoft.SharePoint.PowerShell
Remove-PSSnapin Microsoft.FASTSearch.PowerShell