Like all assemblies which would be added using Add Reference, drop it in your web application's bin subdirectory and you can use it from any code, be it inline with the page or code-behind. Keep in mind that in either case the code is treated the same except code-behind source must be pre-compiled into an assembly for the web application while inline code is compiled upon the first access of the page after the app has been (re)started or the cache temporary page cache cleared. When you compile your code-behind be sure to add a reference (/r switch). If you can't, keep using inline code. Either way, the web reference assembly goes in bin.
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