Working in South America
1.) You need to be the right person with the right experience, in the right place at the right time. Always.
2.) In Argentina, to work, you need a work permit or a permanent residence, the first is possible to obtain if you have an employer willing to take you instead of local talent, the second is more level US green card class of difficulty to obtain. I guess this goes for most South American countries. As was said in other replies, preference goes to locals in a big way. Contacts are invaluable, most, if not all jobs are had via these contacts.
3.) You need to be able to speak Spanish reasonably well. If not, forget it, period.
4.) Exceptions exist, but count your VERY lucky numbers to run into such possibilities.
5.) Remember the sky´s the limit.