I am working on a task in Browser Workbench. I would like to be able to view, lock or unlock segments, but I do not seem to have that option.

In order to be able to lock or unlock segments in Browser Workbench, locking should be first set to in the Advanced setting of the human step(s) you are working on (i.e Translate or Review). The locking should be set to Dynamic Locking.

If the human step is not set to Dynamic Locking, you will not be able to lock or unlock segments and you will also not see that option under Tools.

In order to configure the human step to be able to lock or unlock segments, find the relevant Workflow and open it with the Workflow Editor. Select the human step where you would like to be able to view and lock or unlock segments. Go to the Advanced settings of the human step. 

You will see that locking can be set to No Locking (which is the default status), Dynamic Locking or Static Locking in the Advanced setting of the human step.

• No Locking: Indicates that locks are ignored. Users won't be able to view locked segments or lock and unlock segments. This is the default lock type for human steps.
• Static Locking restricts the user from loving or unlocking segments. However, the translator can view the locked segments or add a comment. This mode is useful when you don't want a translator to modify existing locks, but still be able to view locked segments in Browser Workbench. The Static Locking is usually applied to the Translate step and prevents translators to change segments that have already been reviewed. 
• Dynamic Locking means that this user can view locked segments. The user can also change the status of a locked segment and unlock it or lock it again as required.  The dynamic locking is usually applied to the Review step to allow Reviewers to make changes in the translation (for instance, in ICE Match segments) when required. With this setting, a user will be able to lock or unlock segments in Browser Workbench by selecting one or more segments, go Tools, and selecting Lock Segments or Unlock Segments:

Relevant articles:

WorldServer: After the 'Lock Segments' Automatic Action the segments are still unlocked at the next 'Translate" or 'Review' step
WorldServer - Segments cannot be locked in Online Editor
How are locked Translation Memory Entries created in WorldServer?
WorldServer - after importing a Return package with segments that were locked in Trados Studio, those segments do not display as locked in Browser Workbench