It is not in your path, try '/sbin/ifconfig'. The "system binaries" (/sbin or /usr/sbin) are not normally in a regular users' $PATH. Try 'echo $PATH' to see yours. FYI, It is usually in the root's path though, use 'su -' to switch to the root user. (the dash on the su command reinitialized the enviroment for root, which includes the $PATH variable.)
Another way to find commands is to use locate or find.
'find / -name ifconfig'
The find method actually searches the filesystem, so it takes longer. locate uses a database. Use man (manual pages) to find more info (i.e. ;man find').
I usually change the path, on my logins.
Hope that helps,