How to initiate the Watchdog timer in 89S52?
Remember: nobody reading your posts knows anything at all about you or your level of experience other than what you make explicitly clear in your posts.
IF you want something to be taken into account, it's up to you to make it clear from the outset!
You will also gain more respect - and, so, people will be more inclined to answer helpfully - if you make clear what efforts you've made, and where you're stuck
If you make a post that gives the impression that you haven't even thought to look at the datasheet, that will affect the type of answer that you'll receive.
"what does it mean by 'in sequence'"
In means what it says: a "sequence" is any set of things that happens in order.
So "write 01EH and 0E1h in sequence" means "first write 01EH, then write 0E1H"
"I couldn't see WDTRST sfr in the header of 89S52 in KEIL"
It's just a name - maybe Keil used a slightly different name?
If it's really not there, you can make your own SFR definition as described in the compiler Manual:
http://www.keil.com/support/man/docs/c51/c51_le_sfrs.htm
Sometimes, the compiler vendor "normalizes" names of SFR. But the datasheet will contain the address of the SFR, so you can use this address and look through the Keil header and see if you find any other define that relates to your specific SFR.
In sequence not only means "first a, then b". In this specific situation it means. "first a, then nothing else, then b". You must not do any other write between these two writes for the chip to accept the activation. This is very common in microcontrollers and is intended to protect a crashed program from making spurious writes that may erase memory or similar. In the case of the watchdog, you do not want a crashed program to make spurious writes that results in watchdog kicks, thereby avoiding the saving watchdog reset.