Well, there are a few variables that need to be set before your informatica runtime environment can be considered ready to run jobs...
PM_HOME - must point to the directory where informatica is installed
LIBRARY_HOME or LD_LIBRARY_PATH - depending upon whether you are in windows world or unix (respectively), this variable should include library files from informatica. Typically this should include $PM_HOME/bin
PATH - this must include the executables from informtica as well as any other subsystems you might be accessing.
Its possible that these variables are not setup/created in windows/unix when you first install the informatica software, don't hesitate in creating them afresh. However, if they already exist (e.g. PATH would exist already), make sure you append (and not replace) the new values.