Usually you can just restart SSMS and it works, but this is a known issue:

http://connect.microsoft.com/SQLServer/feedback/details/557402/ssms-can-no-longer-create-or-edit-job-steps

And depending on which version you are on (the name of the question and the forum don't match) here are the fixes:

SQL Server 2008 R2 SP1

or

SQL Server 2008 SP3