when installing adobe suites, you can pick and choose which programs you want installed. you either failed to select it upon initial installation or possibly uninstalled it after the fact. or maybe something else happened.
in any case, to install now, start your cs5.5 installation file(s) and you will again be given the choice of which program(s) to install. tick acrobat X and go.
Downloads available:
Download and installation help for Adobe links
Download and installation help for Prodesigntools links are listed on most linked pages. They are critical; especially steps 1, 2 and 3. If you click a link that does not have those steps listed, open a second window using the Lightroom 3 link to see those 'Important Instructions'.