"How can I provide logical high without connecting it to +5V ? Do you mean that I should connect it through a resistor?"

Yes a resistor is what is implied. Normally called a pull-up resistor. For most derivatives, a value of 4.7 kOhm to 10 kOhm should be OK. But suggest refer to the data sheet of **your** derivative and cross check this before confirming the value.

"One thing I find different from datasheet is that you say that P2.6 and P2.7 both should be high. Is this an error in datasheet ? "

Yes. Both P2.6 and P2.7 must be high. I dont exactly remember the source of this information, but after doing this plus using a reset controller have not faced any problems with ISP.

Raghu