Alright, sorry it took me so long, but you should have the option of installing three version, stand alone, dxi and vst. What you're looking for is the Vst - this i think is the most compatible with fl studio. After installing the files, they should go to program files>steinberg plugins.
Now open fl studio and go to options and then folder settings.
look carefully for extra plugin directory - and click the folder button and input the folder which you installed the sample tank, by default it should be c>program files>steinberg plugins.
Now close and go to channels>add one> more and you should a list of plugins, now look at the bottom for a small button that searches for new plugins, when you click on it, its gonna say fast scan (recommended)
click on it and it'll search the folder that put in earlier for extra plugins. If all is well you should see sample tank come up in red writing - just check the box to the left of it. This makes sample tank a channel that fl can control. To use it, just add it like you would the dx10 or pluck plugin, as many times as you want. But bare in mind you gotta register and downlaod sounds for it - this i am convinced you can figure out.