Once in the pack detail view, you will need to click on the "Open folder" button, this will then open Windows Explorer to the directory that your pack instance is installed in.
There can be 2 different logs here that are useful, there might be a "crash-reports" folder. If there is the newest .txt file will be the one that you want. If there is not a "crash-reports" folder then you will want to go into the "logs" folder and find the file "latest.log". Once you have found the log file, you will need to open it in a text editor, such as notepad.
Once you have your log open, right click and click "Select All" (or ctrl+A) and then right click again and click "Copy" (or ctrl+C)
The contents of that log is now stored in your clipboard, you will now need to go to https://paste.feed-the-beast.com and paste it into the text area (either right click and click "paste" or ctrl+V). Then click on the "Create" button to finish uploading the log, then copy the url that it takes you to and share that with us.