Ifx +odbc +lock

I'm using VB6 as a client program that access data stored in a Informix On-line 5.10 database through ODBC. I've used Informix Client and the driver that comes with it's latest version (for me 2.5). I can do almost anything but I simply can not lock a single row (neither a page) from the program so I can prevent concurrent problems. What's the thing I should do, or the point I'm mising that let me use locking against these Informix database. BTW, I can use transaction, I point that I think could be involved in solving the problem. I also use Informix star product for accessing the information in the unix boxes (the Informix database) and tcp/ip as the protocol underlying. TIA