To expand a bit on what bersl2 said...
Type echo $PATH. You need to get the directory where acroread lives in there. I don't know what directory that is - probably /usr/local/acroread or something similar. Browse around until you figure it out. Once you know the directory, check out /home/yourname/.bashrc or /home/yourname/.bash_profile. Depending on which distro you use, one of those files (try .bashrc first) will have some lines to set your path. Add the directory in there, then open a new terminal and you should be set.