To answer your question, yes, I-DEAS does have both a macro environment (which if I recall correctly is simply a C-structure along with I-DEAS specific commands), plus a more in-depth set of API's(OPEN I-DEAS/OPEN TDM, etc), which I don't know too much about but think they also (at least used to) conform to C/C++ conventions, as this is what most of I-DEAS was written in [except for the Analysis engines which tend to stay Fortran for obvious reasons] (note that these later do typically require additional licenses, etc, but are meant more for full-up ERP/PDM/CAE integrations, etc, than for typical end-user use)
You can consult the on-line help for examples on the 'Program Files' (as the macros are typically referred to), etc, but if you have something specific you'd like to automate, you can simply go to File/Program File(s) and tell the system to start recording (your actions in the software).
I-DEAS then records all your clicks/picks/etc in I-DEAS and outputs them to the filename that you specify (quite a handy thing for creating program templates as well).
This should get you enough to get started.
Also, if you want to have a program file execute each time you start I-DEAS you can rename that file (I think it's something like autorun.prg but should be able to look that up) and place it in the I-DEAS start directory, which is [typically] where ever I-DEAS stores your model files or as otherwise set in the system or your user Param file.
Hope this helps...