I assume that the crew in ISS can manipulate the minisats manually before they are launched, so they must be launched through some kind of airlock. Was this airlock designed for this purpose, or do they make use of an ordinary airlock for astronauts?
There are two CubeSat deployers on the ISS, the JEM (Japanese Experiment Module) Small Satellite Orbital Deployer built by JAXA, and NanoRacks CubeSat Deployer;
J-SSOD is installed on the Multi-Purpose Experiment Platform (MPEP) of the extended slide table in the Kibō module's airlock where two its deployment slots are loaded with up to three CubeSat units (10 cubic centimeters) each, the slide table is then extended out of the airlock and launches CubeSats:
Japan Aerospace Exploration Agency astronaut Aki Hoshide, Expedition 33 flight engineer, works near the airlock in the Kibo laboratory of the International Space Station. The Small Satellite Orbital Deployer (SSOD) previously installed on the Multi-Purpose Experiment Platform (MPEP) is visible in the airlock.
NRSCD, capable of deploying up to 48 units (8 slots with up to 6U each), is also accessible via the Kibō airlock but attaches to the Remote Manipulator System (JEMRMS) robotic arm which then grabs it and CubeSats are deployed from it. This video is made by NanoRacks and it is promotional in nature, so terribly sorry for that (full disclosure: I have nothing to do with NanoRacks), but it's the best I could find and showing deployment of CubeSat satellites from NRCSD attached at the end of JEMRMS:
So both small satellite deployers currently available on the station are accessible via the Kibō airlock, which itself is too small to be used for egress and ingress of astronauts during EVA (Extra-Vehicular Activity), and is exclusively used to access externally, on Exposed Facility (EF) mounted experiments, deploy SmallSats (with the two deployers mentioned), and similar, where MPEP and JEMRMS serve as means of getting hardware and experiments into the airlock and back out again. Its dimensions are 576x830x800 mm, it has less than 600 W of peak power available to it, and it is attached to the Kibō's Pressurized Module (PM).
To my knowledge, Kibō airlock was always attached to the Kibō PM, since it was launched to the station on May 31, 2008 aboard STS-124. Actually, even before, as mentioned on this page that's describing airlock's functional test (test was already conducted inside the flight model of Kibō's PM, in August 1999).
This image explains its use (from JAXA's page on Kibō Structure - Airlock):
This isn't really an answer but offered up as a supplement to Tidal Wave's answer - I can't post an image as a comment. This picture was taken as the payload bay doors were being closed on May 10, 2008 in prep for the STS-124 launch, and shows the airlock being present between the JEMRMS (robotic arm) and the four-pronged connector that would hold the exposed payload facility in the future. Feel free to take the image and incorporate it into the other answer.
This image also shows the enormous size of this payload.