At times I import a midifile and tweak it ... Most of the time when I export a midifile to mp3 there is no problem. But a couple times lately, when I click to export I get this box message and can not go any further to export the file. The message reads ... "the specific selection did not contain any audio data. Please ensure that the selection contains the audio data you wish to bounce" ... Anyone else had to deal with this? Not sure what steps to take to correct this and allow me to export midifile to mp3 like I usually do.

This would be because all of your MIDI tracks are not configured so that their MIDI data gets sent to any soft synths. 

Remember, a MIDI track is simply a set of instructions that tells a synth (be it hardware synth or softsynth) what notes to play , how long to play them, how loud and what-not.  It could also contain controller data like brightness, pitch bend, volume, etc.  The MIDI track by itself does not contain any sounds on its own.

So what you have to do when you import your MIDI file is to make sure that some or all tracks are set to drive a softsynth.  For General MIDI files, a common synth to use for this purpose is TTS.  This is a multi-timbrel synth that has the General MIDI sound set that should work for most MIDI files.

Of course, you could also load a MIDI file directly rather than importing it.  That way you will retrieve tempo data as well.

Thanks promidi ... I do go through the steps of inserting the TTS-1. Most of the time the midifiles are ok to be exported after I adjust them. How to load a midifile directly rather than importing it? What I find is that with a few midifiles that I've got from the web, as soon as I've made some adjustment like deleting some part ... then I get the message that I stated above. It's an odd thing to happen but only with a few files.

You may check if the track where TTS1 is wasn't incidentally deleted during editing. Also remember you have to always select the TTS track (the one with synth/audio outputs) along with your midi selection when exporting midi to audio/mp3.

I cover how to OPEN midi files and use TTS-1 in my tuorial first part.  Never import midi files, always open them. Import is only used for grabbing a midi track or loop. 
See my signature for the link. 

thanks for the advice everyone ... will check out the tutorial from Cactus Music ...