ASIS is among the most difficult packages to create as it needs parts of the compiler sources in order to read the semantic tree files which are created by the compiler. The normal compiler installation won't actually install those sources - you will have to grab them from the GCC source tree and copy then to ASIS source tree.
The upcomming r3 distribution will solve that. r3 will supply an additional "devel" package which will contain sources needed.
Note that the ASIS packages for all compiler versions (including the GNAT/GCC FSF versions) are based on source code that is GPL without the generic/linking exception. Distribution of binaries based on this ASIS library will need to comply with the terms of the GPL.
The GNAT ASIS project hosts the development tree for ASIS but all binary distributions for that project are currently being done through the GNU Ada project.