I am Pete, and I'll be happy to assist with your Excel question.
To use the Mid function, enter the cell reference, then the character you want to start from, and finally the number of characters you want to retrieve.
For example, if cell A1 contained abcdefghij and you wanted to retrieve characters 4-6, your formula would be =MID(A1,4,3) and the output for this formula would be def.
Note that if there are any spaces or symbols in the cell, these will be counted as characters, and may affect the result.
If you would like me to have a look at any specific problems you are experiencing on a spreadsheet, you can try attaching it here using the paperclip icon, or you can upload it to a filesharing website, eg. www.wikisend.com