If I understand your post correctly, you have realised that the problem is one of changing the association of these attached files from the trial version to the professional version of Excel. If this is correct, I sympathise - it is not that obvious.
I don't know if you are trying to run these files directly from the attachment in the e-mail or if you are downloading them first.If you are trying to run them, download one to a location where you can find it - 'Downloads' will do fine but it is your choice. Then use Windows Explorer to navigate to where you downloaded this file to. When you have found it, left click on it once, to select it, then RIGHT click on it, and in the diaalogue box that pops up you will see 'Open with...' . This will produce another dialogue box which has an Excel icon in it. DON'T click on this. In the bottom right hand corner you will see a box marked 'Browse' - click on this instead. Now all you have to do is find the Excel.exe file for Office 2010 !
This lives inside either 'Program files' (32 bit systems) or 'Program files(x86)' (64 bit systems). Click on whichever of these is appropriate. Then click on 'Microsoft Office'. Inside this you you may see two or three folders called 'Office x, and y, and z'. I am guessing slightly here, but I know Office 2003 is 'Office 11' and Office 2007 is 'Office 12' so Office 2010 is probably 'Office 13'. Click on this to open it and you will see the file 'Excel.exe' or just 'Excel' depending on how Explorer is set up, but in either case it will have a small version of the Excel icon beside it. Click on this to select it, then tick the box on the opposite side of the dialogue box from 'Browse'. It is marked 'Always use the selected program to open this type of file.
Exit from this dialogue box and double click on the file you downloaded and it should open in your version of Office 2010 Pro.