Google Chrome

1. Make sure that the Bookmarks Bar is visible. If it is not choose Always show bookmarks bar from the Tools menu
2. Drag MWHelper Switch Profile to your Bookmarks Bar

Firefox

1. Drag MWHelper Switch Profile to your Bookmarks and add where you wish
2. If you drag it to your Bookmarks Toolbar instead remember you can access it when this toolbar isn't visible by going to Bookmarks > Bookmarks Toolbar

Opera

1. Make sure that the Personal bar is checked under View > Toolbars
2. Drag MWHelper Switch Profile to your Personal bar

Safari

1. Make sure that the Bookmarks Bar is visible. If not, go to the menu View > Show Bookmarks Bar
2. Drag MWHelper Switch Profile to your Bookmarks Bar

Internet Explorer

1. Right click MWHelper Switch Profile and click Add to Favorites and add where you wish
2. If you get the message You are adding a favorite that might not be safe. Do you wish o continue? just click Yes

In Internet Explorer 8 you will probably get the message Internet Explorer had modified this page to help prevent cross-site scripting. Click here for more information.... You can resolve this by disabling the XSS filter but enable this later if you surf to new/unknown sites.
The XSS filter is disabled by doing the following: click Tools - Internet Options, then select the Security tab, click Custom level... and scroll nearly to the bottom where you have an option Enable XSS filter that is normally set to Enable. Select Disable confirm the change by clicking Yes, then click OK and OK again. You should now have no problems using the bookmarklet.
As mentioned before, for your safety, enable this before surfing to unknown sites!