Question i need adapter to allow RF modulator and Roku Express+ to be connected to older tv that has 1 RCA hookup

If the TV has an RCA composite video input it should have one or two RCA audio inputs too.
You got the correct Roku that has composite video and audio outputs that should work right into the TV. No modulator needed. You will need the cable that came with the Roku that plugs into the AV output on the back. Better picture this way.
If the TV has a single F connector (this has a threaded outer barrel where the RCA is smooth) that is for an antenna. Then you connect the AV cables to the modulator which connects to the antenna input. Tune to the channel that the modulator is set to output to. Usually channel 3 or 4.
If you only have 1 RCA audio input (TV or modulator) then it doesn't matter which RCA audio channel you use.