I don't remember which spelling I was told, but always use
install. I recently discovered the expression
a new instalment and my dictionary indicates the spelling
installment in the US, which led me to check whether
instal also exists. Surprisingly to me
instal are listed equivalent and without reference to a specific country.
I'm curious about the use of each spelling in British and American English. By the way, my Firefox spelling dictionary redlines the single l version as I write this question.