The principle is:
$Env:variablename
So you might try:
cd $Env:Programfiles
or to temporarily switch working directory to %Programfiles%\MyApp:
Push-Location -Path "$Env:Programfiles\MyApp"
#
# command execution here
#
Pop-Location
To list all environment variables you could do:
Get-ChildItem Env:
or use the convenient alias:
ls env: