I’ve installed tonight’s nightly build (2018-03-09) but still cannot find the SlicerIGT extension. Also fyi, when I type in text in the search bar in the Extensions Manager, sometimes the characters disappear and I have to re-type.
Slicer nightly (preview) build version is going through major changes in these months, so sometimes certain extensions may fail to build for a few days. Either use latest stable version, or download Slicer from a few days before, or wait a few days for the extension to become available again.