There should be no need to manually truncate a log file when using SQL Server, as that functionality is controlled via SQL Server itself. The log file is truncated automatically at specified checkpoints (every 5 minutes, for example) when SQL is running in Simple recovery mode. If SQL is running in FULL recovery mode, the log is never truncated until you manually execute a transaction log backup via SQL (not the same as a backup via the Blackbaud Management Console).
If the log file is growing too large, then verify the recovery model the database is set to run in - is it full or simple? For more information on this setting, review How to change the database from Simple Recovery Mode to Full Recovery Mode.
If it is set to full recovery mode, then the transaction log file will grow and never shrink unless you explicitly configure regular transaction log backups. If you are not making regular transaction log backups, or do not know how to, then the best option is to change the recovery model to SIMPLE. This will allow SQL to manage the log file size, and it should automatically flush the data from the log to the database, shrinking the log file in the process and maintaining its size so that it never grows too large again.
Please contact the appropriate software vendor or IT professional for assistance with this process or issue, which is beyond Blackbaud's scope of support.