First, I'll say I'm much more familiar with Wavepad than Mixpad, so maybe Mixpad can do more than I'm aware of.
Anyway, Wavepad will do everything you need, and Mixpad will only do some of what you need. But Mixpad can be more convenient than Wavepad for mixing files that have already but cut to size, equalized, noise-reduced, etc. In Mixpad you can slide files' relative positions around and adjust volume levels and fades.
But first you have to prepare your files in Wavepad. To convert tapes to digital, you will have to hook up a tape player to the line input of your computer (line in). Then hit the record button in Wavepad a couple times and play your tape player. Try recording to make sure the sound levels are proper and the correct input is being recorded (you can adjust the sound levels and inputs in Wavepad and Windows). If your tape player has line-level output jacks (line out), that might result in better quality, but if not, you can hook up to the headphone out jack of the tape player. After you've adjusted everything (this can take some time), rewind your tape and let Wavepad record it.
After you've recorded a tape you will see the waves in Wavepad. You will probably want to use the amplify function in Wavepad to make them louder. Then you can try noise reduction and the equalizer to improve the sound of the tapes.
Use Wavepad for editing each file you will want to include in the mix. Mixing can also be done in Wavepad (and done very precisely) using the "paste mix" function, but you can't re-arrange your mix as easily in Wavepad, and you have to create any independent fades before you paste files together. You can only re-arrange your mix by clicking 'undo' and pasting again. In Mixpad you can slide 2 or more files' relative positions around and can make independent fades and volume adjustments, but you can't do much other editing.
As with any software, you'll have to play with the programs for a while to really understand them.
