ErrorLevel is not %ERRORLEVEL% . This is probably the first one you should read.
Next is the usage of the ERRORLEVEL statement. http://support.microsoft.com/kb/69576
The following table shows the effect of statement when writing your batch scriipt.
Statement Algebraic Equivalent. IF ERRORLEVEL 5 ... IF E = 5 OR E > 5 THEN ... IF NOT ERRORLEVEL 6 IF E < 6 THEN ...
Here is a sample using findstr and error level. Findstr returns 0 if it successfully finds any occurrence.
findstr -sip Failed log.txt > NULL IF NOT ERRORLEVEL 1 ( echo Found. ) else ( echo Not Found. )