There are a number of ways that this could be set up, but first you need to disable the cabinet of your amp sim, and this varies by amp sim. For Guitar Rig 5, you can set the Control Room Pro up for “No Cabinet,” or you can just turn it off (Power Switch). For TH3, don't add the matching cab or disable it if it's there. (EDIT: As pointed out below by vintagevibe, TH3 can indeed load IRs. Load Cabinet IR and edit its Properties.)
Secondly, you'll need to load some sort of convolver and import the IR file of your choice (and there are a huge number of choices!). Convolvers vary all over the map. Ownhammer has 200 msec and 500 msec IR files, so if you want the longer ones (not sure that's necessary) you'll need a convolver that doesn't cut off the IR early. If you want to play live, you'll want one that has low latency. And so on....
As a special note for users of Samplitude: You can use the Room Simulator included in the DAW to load the IR, but be sure to turn the dry signal all the way down so that all the signal goes through the convolver --- all wet.
Similarly, Reaper has ReaVerb (haven't tried it myself). Here also disable the Dry signal or turn it way down.
Some amp sims (e.g. Revalver 4) allow you to load an arbitrary IR into the cabinet simulation part of the amp sim.
Kazrog's Recabinet 5, itself a cabinet sim with a library of IRs, allows you to load additional arbitrary IRs in addition to loading Recabinet files.
Hope this helps.
Regards,
Dave Clark
PS to Dave: Thanks for posting the clarification. I was in "Wow!" mode and not thinking clearly....