It's not possible to record a voiceover in parts if you are recording it in VideoScribe. To record in parts you would need to use an external audio recording application such as Audacity (free), export as an MP3 and import into VideoScribe. See Record a voiceover for more information.
The plus and minus characters may appear small on the canvas when you add them, especially when using the 'Basic' font, so you need to make them larger so they are visible. Try to keep the zoom level close to 100% when adding and resizing the text. You can also use different fonts (such as Arial) by importing a font into VideoScribe. See also Add mathematical formulae.