A 7490 contains a divide by two counter and a divide by five counter. As for
the divide by two counter the input is on pin 14 and the output on pin 12.
Of course you will have to satisfy the preset and clear inputs 2,3,6 and 7.
Keep them low. It's also good practice to keep the unused inputs at a fixed
value. So keep pin 1 high, preferrable via a 10k resistor just to stay on
the safe side. All of this can be checked by reading the data sheet. A
little Google and you know all you need and more.
Using a DECADE counter to divide by two? Talk about overkill...
All you need is a D-type flip-flop. A 7474 will do nicely. Simply
wire the Q-not output to the 'D' input, send your input to the CLK lead,
and take your output from the Q output. Simple, neat, reliable.
You can download the manufacturers specification sheet from rswww.com.
You'll have to register, but that's free. Just follow the products to the
7490, and click on the tech info tab. You'll get a choice of half a dozen
manufacturers data sheets.
I use RS this way for getting info all the time. They are also an excellent
source of parts and, although located in the UK, will ship transatlantic at
very low cost.
There's also a very good explanation of the whole thing in "The Art of
Electronics, by Horowitz and Hill, which should be available in the
technical reference section of any central library close to you.
If all else fails, e-mail me and I'll scan the relevant pages and send them