In order to query the log, in addtion to the builtin query viewer of ISA, you can install SQL client tools, then use SQL Analyzer to connect to ISA-SERVER\MSFW (replace ISA-SERVER with your actual server name) and query the log. See the example here.
The builtin query viewer of ISA can show only the first 10,000 log records.
In addition, you can use the script to show the log entries. A sample script can be downloaded here. It can query the last 30 days of log. Just download it, rename it to a *.vbs file, and run it on your ISA Server. Make sure to change the first 2 input data (LogDate and ClientUserName) in the script file to match your actual data before runing. And, don't forget that the script syntax is cscript LogQuery.vbs [LogData.txt]
Below is a sample result of a script query.