Go to Targets, Host. Select the Host you want a log file to be monitored. Select Metrics and Policy Settings. In "View" select either All Metrics or Metrics with Thresholds and you should see the metrics "Log File Pattern Matched Line Count". In the Edit column select the pencils for the mentioned metric. Select Add and now you can put the directory + log file name in "Log File Name" and a search pattern (Error or Severe or CRITICAL or WARNING) in "Match Pattern in Perl". If you want a Warning to be reased if the Pattern is matched fill in a 0 in the column "Warning Threshold".
The same goes for Critical, put a 0 in "Critical Threshold".