You're not going to be able to repair this - that log file is now toast.

Here's what I would try (follow these instructions at your own risk):

1) shutdown the server

2) copy off the msdb files

3) delete the msdb log file

4) start the server

5) emergency mode repair of msdb (you may need T3608 for this - don't remember)

6) make sure all your jobs and SSIS packages are still there

OR

1) script out all information from msdb

2) create a new msdb

3) reinsert all information into the new msdb

If either of these are beyond the scope of your comfort/expertise, you need to get someone else to help you.

No matter which you choose, you need to analyze the IO subsystem to find out why the log file became corrupt.